multiple -m

sync, assist, import: Allow -m option to be specified multiple times, to
provide additional paragraphs for the commit message.

The option parser didn't allow multiple -m before, so there is no risk of
behavior change breaking something that was for some reason using multiple
-m already.

Pass through to git commands, so that the method used to assemble the
paragrahs is whatever git does. Which might conceivably change in the
future.

Note that git commit-tree has supported -m since git 1.7.7. commitTree
was probably not using it since it predates that version. Since the
configure script prevents building git-annex with git older than 2.1,
there is no risk that it's not supported now.

Sponsored-by: Nicholas Golder-Manning on Patreon

doc/todo/multiple_-m.mdwn

@@ -1,2 +1,4 @@
 git-annex sync etc -m should be able to be specified multiple times. In git
 commit, multiple -m can be used to make a multiparagraph commit. --[[Joey]]
+
+> [[done]]