Allow the branch name updated by sync-webkit-git.py to be overriden using the git config in the WebKit directory.
Allow the branch name updated by sync-webkit-git.py to be overriden using the
git config in the WebKit directory.

My scenario is that I have the same git repository (using git-new-workdir) under
two different chromium enlistments. (Yes, even with git, I find it useful to
have more than one enlistment at times.)

BUG=None
TEST=Did a run of the tool without the config set and with it set to an
     existing branch and a non-existing branch.

Review URL: http://codereview.chromium.org/3239002

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@57690 0039d316-1c4b-4281-b951-d872f2087c98

keishi/chromium,rogerwang/chromium,patrickm/chromium.src,junmin-zhu/chromium-rivertrail,pozdnyakov/chromium-crosswalk,nacl-webkit/chrome_deps,ondra-novak/chromium.src,Jonekee/chromium.src,mohamed--abdel-maksoud/chromium.src,mohamed--abdel-maksoud/chromium.src,fujunwei/chromium-crosswalk,nacl-webkit/chrome_deps,Chilledheart/chromium,ChromiumWebApps/chromium,dushu1203/chromium.src,ChromiumWebApps/chromium,ltilve/chromium,nacl-webkit/chrome_deps,ondra-novak/chromium.src,hujiajie/pa-chromium,Jonekee/chromium.src,zcbenz/cefode-chromium,littlstar/chromium.src,junmin-zhu/chromium-rivertrail,dednal/chromium.src,PeterWangIntel/chromium-crosswalk,Just-D/chromium-1,robclark/chromium,Jonekee/chromium.src,TheTypoMaster/chromium-crosswalk,markYoungH/chromium.src,Pluto-tv/chromium-crosswalk,zcbenz/cefode-chromium,pozdnyakov/chromium-crosswalk,robclark/chromium,PeterWangIntel/chromium-crosswalk,littlstar/chromium.src,markYoungH/chromium.src,chuan9/chromium-crosswalk,markYoungH/chromium.src,hgl888/chromium-crosswalk-efl,Fireblend/chromium-crosswalk,keishi/chromium,chuan9/chromium-crosswalk,markYoungH/chromium.src,PeterWangIntel/chromium-crosswalk,Pluto-tv/chromium-crosswalk,mogoweb/chromium-crosswalk,M4sse/chromium.src,robclark/chromium,Fireblend/chromium-crosswalk,ondra-novak/chromium.src,hujiajie/pa-chromium,jaruba/chromium.src,zcbenz/cefode-chromium,M4sse/chromium.src,hgl888/chromium-crosswalk-efl,markYoungH/chromium.src,dushu1203/chromium.src,Pluto-tv/chromium-crosswalk,hgl888/chromium-crosswalk,nacl-webkit/chrome_deps,ChromiumWebApps/chromium,krieger-od/nwjs_chromium.src,Fireblend/chromium-crosswalk,pozdnyakov/chromium-crosswalk,ChromiumWebApps/chromium,axinging/chromium-crosswalk,junmin-zhu/chromium-rivertrail,Just-D/chromium-1,bright-sparks/chromium-spacewalk,TheTypoMaster/chromium-crosswalk,hgl888/chromium-crosswalk-efl,Just-D/chromium-1,anirudhSK/chromium,jaruba/chromium.src,Just-D/chromium-1,pozdnyakov/chromium-crosswalk,krieger-od/nwjs_chromium.src,dushu1203/chromium.src,robclark/chromium,robclark/chromium,junmin-zhu/chromium-rivertrail,zcbenz/cefode-chromium,patrickm/chromium.src,littlstar/chromium.src,mohamed--abdel-maksoud/chromium.src,Pluto-tv/chromium-crosswalk,markYoungH/chromium.src,rogerwang/chromium,TheTypoMaster/chromium-crosswalk,dushu1203/chromium.src,anirudhSK/chromium,PeterWangIntel/chromium-crosswalk,robclark/chromium,Pluto-tv/chromium-crosswalk,PeterWangIntel/chromium-crosswalk,Pluto-tv/chromium-crosswalk,chuan9/chromium-crosswalk,M4sse/chromium.src,M4sse/chromium.src,jaruba/chromium.src,crosswalk-project/chromium-crosswalk-efl,crosswalk-project/chromium-crosswalk-efl,axinging/chromium-crosswalk,hujiajie/pa-chromium,ChromiumWebApps/chromium,hgl888/chromium-crosswalk-efl,Fireblend/chromium-crosswalk,littlstar/chromium.src,TheTypoMaster/chromium-crosswalk,ltilve/chromium,krieger-od/nwjs_chromium.src,Chilledheart/chromium,krieger-od/nwjs_chromium.src,timopulkkinen/BubbleFish,dushu1203/chromium.src,Just-D/chromium-1,mogoweb/chromium-crosswalk,hgl888/chromium-crosswalk,bright-sparks/chromium-spacewalk,patrickm/chromium.src,nacl-webkit/chrome_deps,ChromiumWebApps/chromium,dushu1203/chromium.src,jaruba/chromium.src,ltilve/chromium,anirudhSK/chromium,dednal/chromium.src,TheTypoMaster/chromium-crosswalk,dednal/chromium.src,rogerwang/chromium,Chilledheart/chromium,crosswalk-project/chromium-crosswalk-efl,keishi/chromium,PeterWangIntel/chromium-crosswalk,Pluto-tv/chromium-crosswalk,crosswalk-project/chromium-crosswalk-efl,anirudhSK/chromium,hgl888/chromium-crosswalk-efl,rogerwang/chromium,axinging/chromium-crosswalk,fujunwei/chromium-crosswalk,PeterWangIntel/chromium-crosswalk,hgl888/chromium-crosswalk,rogerwang/chromium,fujunwei/chromium-crosswalk,rogerwang/chromium,Chilledheart/chromium,rogerwang/chromium,anirudhSK/chromium,patrickm/chromium.src,keishi/chromium,markYoungH/chromium.src,ltilve/chromium,hujiajie/pa-chromium,junmin-zhu/chromium-rivertrail,Fireblend/chromium-crosswalk,littlstar/chromium.src,patrickm/chromium.src,chuan9/chromium-crosswalk,timopulkkinen/BubbleFish,M4sse/chromium.src,dednal/chromium.src,Just-D/chromium-1,ChromiumWebApps/chromium,zcbenz/cefode-chromium,junmin-zhu/chromium-rivertrail,pozdnyakov/chromium-crosswalk,mohamed--abdel-maksoud/chromium.src,dednal/chromium.src,Jonekee/chromium.src,patrickm/chromium.src,junmin-zhu/chromium-rivertrail,nacl-webkit/chrome_deps,dednal/chromium.src,Chilledheart/chromium,robclark/chromium,ltilve/chromium,junmin-zhu/chromium-rivertrail,Fireblend/chromium-crosswalk,M4sse/chromium.src,hgl888/chromium-crosswalk,hgl888/chromium-crosswalk,anirudhSK/chromium,mogoweb/chromium-crosswalk,mogoweb/chromium-crosswalk,axinging/chromium-crosswalk,hgl888/chromium-crosswalk-efl,rogerwang/chromium,mohamed--abdel-maksoud/chromium.src,axinging/chromium-crosswalk,timopulkkinen/BubbleFish,anirudhSK/chromium,dushu1203/chromium.src,dushu1203/chromium.src,mohamed--abdel-maksoud/chromium.src,dushu1203/chromium.src,junmin-zhu/chromium-rivertrail,krieger-od/nwjs_chromium.src,keishi/chromium,dednal/chromium.src,keishi/chromium,Just-D/chromium-1,robclark/chromium,Chilledheart/chromium,mohamed--abdel-maksoud/chromium.src,robclark/chromium,markYoungH/chromium.src,jaruba/chromium.src,jaruba/chromium.src,littlstar/chromium.src,nacl-webkit/chrome_deps,dushu1203/chromium.src,fujunwei/chromium-crosswalk,krieger-od/nwjs_chromium.src,fujunwei/chromium-crosswalk,hgl888/chromium-crosswalk,mogoweb/chromium-crosswalk,jaruba/chromium.src,Jonekee/chromium.src,pozdnyakov/chromium-crosswalk,hujiajie/pa-chromium,hujiajie/pa-chromium,Pluto-tv/chromium-crosswalk,bright-sparks/chromium-spacewalk,mogoweb/chromium-crosswalk,anirudhSK/chromium,Fireblend/chromium-crosswalk,axinging/chromium-crosswalk,rogerwang/chromium,anirudhSK/chromium,zcbenz/cefode-chromium,ChromiumWebApps/chromium,M4sse/chromium.src,ChromiumWebApps/chromium,hgl888/chromium-crosswalk,keishi/chromium,krieger-od/nwjs_chromium.src,fujunwei/chromium-crosswalk,pozdnyakov/chromium-crosswalk,dednal/chromium.src,Chilledheart/chromium,krieger-od/nwjs_chromium.src,pozdnyakov/chromium-crosswalk,M4sse/chromium.src,Jonekee/chromium.src,robclark/chromium,hgl888/chromium-crosswalk-efl,pozdnyakov/chromium-crosswalk,Jonekee/chromium.src,keishi/chromium,timopulkkinen/BubbleFish,fujunwei/chromium-crosswalk,junmin-zhu/chromium-rivertrail,ChromiumWebApps/chromium,jaruba/chromium.src,patrickm/chromium.src,dednal/chromium.src,mohamed--abdel-maksoud/chromium.src,ondra-novak/chromium.src,littlstar/chromium.src,hgl888/chromium-crosswalk-efl,nacl-webkit/chrome_deps,M4sse/chromium.src,TheTypoMaster/chromium-crosswalk,Pluto-tv/chromium-crosswalk,pozdnyakov/chromium-crosswalk,pozdnyakov/chromium-crosswalk,bright-sparks/chromium-spacewalk,hgl888/chromium-crosswalk-efl,bright-sparks/chromium-spacewalk,chuan9/chromium-crosswalk,krieger-od/nwjs_chromium.src,Jonekee/chromium.src,ondra-novak/chromium.src,mogoweb/chromium-crosswalk,ChromiumWebApps/chromium,PeterWangIntel/chromium-crosswalk,timopulkkinen/BubbleFish,fujunwei/chromium-crosswalk,nacl-webkit/chrome_deps,TheTypoMaster/chromium-crosswalk,junmin-zhu/chromium-rivertrail,jaruba/chromium.src,Chilledheart/chromium,hgl888/chromium-crosswalk,mohamed--abdel-maksoud/chromium.src,markYoungH/chromium.src,crosswalk-project/chromium-crosswalk-efl,bright-sparks/chromium-spacewalk,PeterWangIntel/chromium-crosswalk,hujiajie/pa-chromium,patrickm/chromium.src,mogoweb/chromium-crosswalk,ltilve/chromium,bright-sparks/chromium-spacewalk,bright-sparks/chromium-spacewalk,Jonekee/chromium.src,timopulkkinen/BubbleFish,mogoweb/chromium-crosswalk,axinging/chromium-crosswalk,chuan9/chromium-crosswalk,hgl888/chromium-crosswalk-efl,keishi/chromium,keishi/chromium,Jonekee/chromium.src,M4sse/chromium.src,nacl-webkit/chrome_deps,zcbenz/cefode-chromium,markYoungH/chromium.src,ondra-novak/chromium.src,keishi/chromium,anirudhSK/chromium,Fireblend/chromium-crosswalk,mogoweb/chromium-crosswalk,hujiajie/pa-chromium,mohamed--abdel-maksoud/chromium.src,chuan9/chromium-crosswalk,chuan9/chromium-crosswalk,jaruba/chromium.src,timopulkkinen/BubbleFish,hujiajie/pa-chromium,axinging/chromium-crosswalk,hujiajie/pa-chromium,ondra-novak/chromium.src,ltilve/chromium,axinging/chromium-crosswalk,zcbenz/cefode-chromium,chuan9/chromium-crosswalk,zcbenz/cefode-chromium,fujunwei/chromium-crosswalk,axinging/chromium-crosswalk,dednal/chromium.src,hgl888/chromium-crosswalk,dushu1203/chromium.src,Chilledheart/chromium,ondra-novak/chromium.src,crosswalk-project/chromium-crosswalk-efl,M4sse/chromium.src,Jonekee/chromium.src,littlstar/chromium.src,dednal/chromium.src,Just-D/chromium-1,timopulkkinen/BubbleFish,Just-D/chromium-1,timopulkkinen/BubbleFish,mohamed--abdel-maksoud/chromium.src,zcbenz/cefode-chromium,ltilve/chromium,anirudhSK/chromium,bright-sparks/chromium-spacewalk,crosswalk-project/chromium-crosswalk-efl,timopulkkinen/BubbleFish,nacl-webkit/chrome_deps,ltilve/chromium,ChromiumWebApps/chromium,markYoungH/chromium.src,krieger-od/nwjs_chromium.src,hujiajie/pa-chromium,zcbenz/cefode-chromium,timopulkkinen/BubbleFish,TheTypoMaster/chromium-crosswalk,jaruba/chromium.src,krieger-od/nwjs_chromium.src,Fireblend/chromium-crosswalk,axinging/chromium-crosswalk,TheTypoMaster/chromium-crosswalk,crosswalk-project/chromium-crosswalk-efl,crosswalk-project/chromium-crosswalk-efl,ondra-novak/chromium.src,anirudhSK/chromium,patrickm/chromium.src,rogerwang/chromium