mv: be quiet about overwriting

When a user asks us to force a mv and overwrite the
destination, we print a warning. However, since a typical
use would be:

  $ git mv one two
  fatal: destination exists, source=one, destination=two
  $ git mv -f one two
  warning: overwriting 'two'

this warning is just noise. We already know we're
overwriting; that's why we gave -f!

This patch silences the warning unless "--verbose" is given.

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