Make minor correctness changes to some "arc patch" command execution
Since I'm in here for PHI1083:
- Add some "--" so we get correct behavior when you have a file named "master", a branch named "README.txt", etc.
- Stop using "%C" unnecessarily.
- Fix some untranslatable strings.
Test Plan: Ran arc patch a couple of times, ran the variations of git commands to catch anything weird with "--" handling.
Reviewed By: amckinley
Differential Revision: https://secure.phabricator.com/D20254