Don't hardcode git directory path.
Don't hardcode git directory path.

The git path is based on the git version, so it might change whenever git is
updated. Run git.bat instead, since that's updated with new git installs and
should always point to the right path.

Review URL: https://codereview.chromium.org/173393002

git-svn-id: bd64dd6fa6f3f0ed0c0666d1018379882b742947@252299 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

svn2github/chromium-depot-tools,svn2github/chromium-depot-tools,svn2github/chromium-depot-tools