Error out in `gclient config` if --name is relative
Error out in `gclient config` if --name is relative

This helps prevent behavior such as setting '--name .' so that the .gclient
file contains '.' as the name of the solution, and the .gclient file ends
up a sibling of the .git file, rather than a sibling of the git checkout's
containing directory.

R=iannucci@chromium.org, thestig@chromium.org

Review URL: https://codereview.chromium.org/1406053003

git-svn-id: fd409f4bdeea2bb50a5d34bb4d4bfc2046a5a3dd@297343 0039d316-1c4b-4281-b951-d872f2087c98

duongbaoduy/gtools,disigma/depot_tools,duanwujie/depot_tools,duongbaoduy/gtools,azureplus/chromium_depot_tools,primiano/depot_tools,hsharsha/depot_tools,duanwujie/depot_tools,hsharsha/depot_tools,CoherentLabs/depot_tools,aleonliao/depot_tools,aleonliao/depot_tools,duongbaoduy/gtools,hsharsha/depot_tools,disigma/depot_tools,azureplus/chromium_depot_tools,CoherentLabs/depot_tools,primiano/depot_tools,azureplus/chromium_depot_tools,duanwujie/depot_tools,disigma/depot_tools,aleonliao/depot_tools,primiano/depot_tools