revert/cherry-pick: work on merge commits as well

Usually you cannot revert a merge because you do not know which
side of the merge should be considered the mainline (iow, what
change to reverse).

With this patch, cherry-pick and revert learn -m (--mainline)
option that lets you specify the parent number (starting from 1)
of the mainline, so that you can:

	git revert -m 1 $merge

to reverse the changes introduced by the $merge commit relative
to its first parent, and:

	git cherry-pick -m 2 $merge

to replay the changes introduced by the $merge commit relative
to its second parent.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

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