Improve argument parsing for "arc patch --revision Dxxx"

Summary: See PHI527. Ref T13116. The --revision flag currently fails if the argument is in the form D123 instead of 123. Normalize monogram arguments.

Test Plan: Ran arc patch --revision Dxxx, arc patch --revision xxx, arc patch --revision xxx --diff yyy, arc patch; got good behavior on the good ones and sensible error messages on the other ones.

Maniphest Tasks: T13116

Differential Revision: https://secure.phabricator.com/D19292