[Mac] Fix official build break where the builders do not have git installed.
[Mac] Fix official build break where the builders do not have git installed.

This fixes the bug two ways to be extra defensive.

BUG=none
TEST=tweak_info_plist succeeds on internal waterfall

Review URL: http://codereview.chromium.org/7037030

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@85782 0039d316-1c4b-4281-b951-d872f2087c98

gavinp/chromium,adobe/chromium,adobe/chromium,gavinp/chromium,yitian134/chromium,gavinp/chromium,ropik/chromium,yitian134/chromium,gavinp/chromium,yitian134/chromium,ropik/chromium,ropik/chromium,gavinp/chromium,yitian134/chromium,adobe/chromium,yitian134/chromium,ropik/chromium,gavinp/chromium,adobe/chromium,gavinp/chromium,ropik/chromium,ropik/chromium,adobe/chromium,ropik/chromium,yitian134/chromium,adobe/chromium,adobe/chromium,adobe/chromium,gavinp/chromium,adobe/chromium,ropik/chromium,yitian134/chromium,adobe/chromium,ropik/chromium,yitian134/chromium,yitian134/chromium,adobe/chromium,yitian134/chromium,gavinp/chromium,gavinp/chromium