git-arc: Add -c flag to patch to commit the change

git arc patch -c D1234 will download differential D1234, try to apply it
to the tree, and if successful will ask phab for the title and
summary. It will construct a commit message with that, the reviewers,
and the differential revision. It also tries its best to deduce the
proper 'author' to use for the commit, and warn if it thinks it has made
a bad choice.

Sponsored by: Netflix
Reviewed by: markj
Differential Revision: https://reviews.freebsd.org/D39992