This uses the same trick as git-new-workdir to reuse an existing git
checkout without interfering with it. However, this makes it only usable
on platforms where os.symlink exists.

BUG=404755

Review URL: https://codereview.chromium.org/1342383002

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@296920 0039d316-1c4b-4281-b951-d872f2087c98