git-arc: Add a diff subcommandHEADmain

Show the differences between local commits and their associated
Phabricator reviews, i.e., what "git arc update" would upload.  For
each commit, the review's current raw diff is applied to the commit's
parent in a temporary index and the resulting tree is compared against
the commit itself.  An empty diff means the commit and the review are
in sync.

This makes it easy to check whether local amendments have diverged
from the posted review before updating it, or to confirm that a
review is current before landing.

Reviewed by:	markj
Differential Revision:	https://reviews.freebsd.org/D58789

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