Also, allow multiple git installations to exist side-by-side.
This makes it easier to revert back to an old version quickly.

BUG=
R=maruel@chromium.org

Review URL: https://codereview.chromium.org/203463004

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@258058 0039d316-1c4b-4281-b951-d872f2087c98