It would fail otherwise when called directly from a git checkout subdirectory.

Updated unit test.

BUG=none
TEST=none

Review URL: http://codereview.chromium.org/6459010

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@74281 0039d316-1c4b-4281-b951-d872f2087c98