rename "match_refs()" to "match_push_refs()"

Yes, there is a warning that says the function is only used by push in big
red letters in front of this function, but it didn't say a more important
thing it should have said: what the function is for and what it does.

Rename it and document it to avoid future confusion.

Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/builtin/send-pack.c b/builtin/send-pack.c
index 87833f4..e0b8030 100644
--- a/builtin/send-pack.c
+++ b/builtin/send-pack.c
@@ -509,7 +509,7 @@
 		flags |= MATCH_REFS_MIRROR;
 
 	/* match them up */
-	if (match_refs(local_refs, &remote_refs, nr_refspecs, refspecs, flags))
+	if (match_push_refs(local_refs, &remote_refs, nr_refspecs, refspecs, flags))
 		return -1;
 
 	set_ref_status_for_push(remote_refs, args.send_mirror,