git-arc: Add list mode support for the update command

This can be particularly useful to do bulk-updates of multiple commits
using the same message, e.g.

git arc update -lm "Move function xyz to libfoo" main..myfeature

Similar to the list mode for the create command, git arc will list all
the candidate revisions with a single prompt. Once that is confirmed,
all the revisions are updated without showing the diffs or pausing
further prompts.

Reviewed by: markj
Differential Revision: https://reviews.freebsd.org/D45050