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

builtin/mv.c

@@ -173,7 +173,8 @@ int cmd_mv(int argc, const char **argv, const char *prefix)
 				 * check both source and destination
 				 */
 				if (S_ISREG(st.st_mode) || S_ISLNK(st.st_mode)) {
-					warning(_("overwriting '%s'"), dst);
+					if (verbose)
+						warning(_("overwriting '%s'"), dst);
 					bad = NULL;
 				} else
 					bad = _("Cannot overwrite");