Make "arc land --merge" an explicit error when targeting a Perforce remote

Summary: Ref T13434. Since "git p4 submit" gets more complicated when submitting merges, and empty merges (as with "--no-ff") seem to vanish, and it's not clear this is desirable or useful anyway, just make the "merge" strategy an explicit error with Perforce remotes.

Test Plan: Ran "arc land --merge ..." in a Git/Perforce repository, got an explicit error. Ran "arc land --squash ...", got existing working behavior.

Maniphest Tasks: T13434

Differential Revision: https://secure.phabricator.com/D20871