This uses grep instead.  Note that just grepping for "--force" matches other
things, e.g. "foo--force--foo", but that's a problem with the original code as
well, I think.

BUG=435729
TEST=Run gclient under msysgit bash.  No error should occur.

Review URL: https://codereview.chromium.org/789353002

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@293340 0039d316-1c4b-4281-b951-d872f2087c98