After crrev.com/896453002, if "git cl diff" ends up having conflict, it
would be cleaned up. However, if "git cl patch" ends up with conflict,
the user should still be able to manually resolve them.

BUG=438362
TEST=tests/git_cl_test.py

Review URL: https://codereview.chromium.org/1091283004

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@295051 0039d316-1c4b-4281-b951-d872f2087c98