.git can be a file that contains actual git-dir.
crup-runner couldn't handle this case as managed checkout.

Review URL: https://codereview.chromium.org/84543002

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@237074 0039d316-1c4b-4281-b951-d872f2087c98