Chrome on iOS downstream repository tracks Chromium via DEPS and wants
to use cpplint canned presubmit check but the cpplint.py errors due at
the include guard as it stops at the inner most git repository.

Change cpplint.py to look for the outermost git repository when looking
for the repository root.

BUG=598090

Review URL: https://codereview.chromium.org/1897153003

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@300085 0039d316-1c4b-4281-b951-d872f2087c98