builtin/merge: honor commit-msg hook for merges

Similar to 65969d43d1 (merge: honor prepare-commit-msg hook, 2011-02-14)
merge should also honor the commit-msg hook: When a merge is stopped due
to conflicts or --no-commit, the subsequent commit calls the commit-msg
hook.  However, it is not called after a clean merge. Fix this
inconsistency by invoking the hook after clean merges as well.

This change is motivated by Gerrit's commit-msg hook to install a ChangeId
trailer into the commit message. Without such a ChangeId, Gerrit refuses
to accept any commit by default, such that the inconsistency of (not)
running the commit-msg hook between commit and merge leads to confusion
and might block people from getting their work done.

As the githooks man page is very vocal about the possibility of skipping
the commit-msg hook via the --no-verify option, implement the option
in merge, too.

'git merge --continue' is currently implemented as calling cmd_commit
with no further arguments. This works for most other merge related options,
such as demonstrated via the --allow-unrelated-histories flag in the
test. The --no-verify option however is not remembered across invocations
of git-merge. Originally the author assumed an alternative in which the
'git merge --continue' command accepts the --no-verify flag, but that
opens up the discussion which flags are allows to the continued merge
command and which must be given in the first invocation.

Signed-off-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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