Support Perforce/Git repositories in "arc land"

Summary: Ref T13434. Detect perforce remotes and use "git p4" commands in place of "git" commands when operating in Perforce mode.

Test Plan:

• Landed "master" onto itself, saw master update.
• Landed "feature1" onto clean "master", saw master update.
• Landed "feature2" onto dirty "master", saw master stay dirty.
• Landed with "--hold", got sensible submit instructions.

Maniphest Tasks: T13434

Differential Revision: https://secure.phabricator.com/D20868