'git rev-parse foo' with foo being any 40 character hex string will succeed.
Strip one char to force git to do a proper hash table lookup.

TBR=bauerb@chromium.org
BUG=
TEST=


Review URL: https://chromiumcodereview.appspot.com/10693008

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@144481 0039d316-1c4b-4281-b951-d872f2087c98