[Proposal] git am --check

This flag would behave similarly to git apply --check, or in other words would exit with a nonzero status if the patch is not applicable without actually applying the patch otherwise.

Rationale: I'm working on an email client which has some git
integration, and when you scroll over a patch I want to quickly test its
applicability and show an indication of the result.

Thoughts on the approach are welcome; my initial naive patch just tried to add --check to the apply flags but that didn't work as I had hoped. Will take another crack at a patch soon(ish).