merge: pass verbosity flag down to merge-recursive

merge: pass verbosity flag down to merge-recursive

This makes "git merge --quiet" really quiet when we call
into merge-recursive.

Note that we can't just pass our flag down as-is; the two
parts of the code use different scales. We center at "0" as
normal for git-merge (with "--quiet" giving a negative
value), but merge-recursive uses "2" as its center.  This
patch passes a negative value to merge-recursive rather than
"1", though, as otherwise the user would have to use "-qqq"
to squelch all messages (but the downside is that the user
cannot distinguish between levels 0-2 if without resorting
to the GIT_MERGE_VERBOSITY variable).

We may want to review and renormalize the message severities
in merge-recursive, but that does not have to happen now.
This is at least in improvement in the sense that we are
respecting "--quiet" at all.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>