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

Khan/arcanist,uber/arcanist,uber/arcanist,aik099/arcanist,codelabs-ch/arcanist,Khan/arcanist,codelabs-ch/arcanist,codelabs-ch/arcanist,aik099/arcanist,aik099/arcanist,phacility/arcanist,phacility/arcanist,codelabs-ch/arcanist,phacility/arcanist,uber/arcanist,Khan/arcanist,codelabs-ch/arcanist,uber/arcanist,aik099/arcanist,phacility/arcanist,phacility/arcanist,Khan/arcanist