rm: resolving by removal is not a warning-worthy event

rm: resolving by removal is not a warning-worthy event

When resolving a conflict on a path in favor of removing it, using
"git rm" on it is the standard way to do so.  The user however is
greeted with a "needs merge" message during that operation:

	$ git merge side-branch
	$ edit conflicted-path-1
	$ git add conflicted-path-1
	$ git rm conflicted-path-2
	conflicted-path-2: needs merge
	rm 'conflicted-path-2'

The removal by "git rm" does get performed, but an uninitiated user
may find it confusing, "needs merge? so I need to resolve conflict
before being able to remove it???"

The message is coming from "update-index --refresh" that is called
internally to make sure "git rm" knows which paths are clean and
which paths are dirty, in order to prevent removal of paths modified
relative to the index without the "-f" option.  We somehow ended up
not squelching this message which seeped through to the UI surface.

Use the same mechanism used by "git commit", "git describe", etc. to
squelch the message.

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