Make minor correctness changes to some "arc patch" command execution

Summary:
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.

Reviewers: amckinley

Reviewed By: amckinley

Differential Revision: https://secure.phabricator.com/D20254