configured for it when figuring out the patch to upload to the try
server.  This prevents git installations where the diff is configured
to use an interactive tool to work without doing nasty workarounds.

BUG=none
TEST=none

Review URL: http://codereview.chromium.org/1958002

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@47661 0039d316-1c4b-4281-b951-d872f2087c98