git-arc: Accept message via -m when updating reviews.

If a -m argument is given to update, it is passed through to arc diff
when updating each review. Note that if an empty message is specified
via -m, arc diff will update the review without adding a note.

If an -m argument is not given, then the user's editor is invoked by
arc to supply a message for each review matching the previous
behavior.

This can be used to simplify the process for updating a set of
reviews, e.g.:

git checkout foo
git rebase main
git arc update -m "Rebase" main..

This will rebase the 'foo' branch and update the reviews for all
commits on the branch without invoking the user's editor separately
for each review.

Reviewed by: markj
Differential Revision: https://reviews.freebsd.org/D37260