match_explicit_lhs: allow a "verify only" mode

The match_explicit_lhs function has all of the logic
necessary to verify the refspecs without actually doing any
work. This patch lets callers pass a NULL "match" pointer to
indicate they want a "verify only" operation.

For the most part, we just need to avoid writing to the NULL
pointer. However, we also have to refactor the
try_explicit_object_name sub-function; it indicates success by
allocating and returning a new ref. Instead, we give it an
"out" parameter for the match and return a numeric status.
Signed-off-by: NJeff King <peff@peff.net>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

remote.c

@@ -1031,10 +1031,12 @@ int count_refspec_match(const char *pattern,
 		}
 	}
 	if (!matched) {
+		if (matched_ref)
 			*matched_ref = matched_weak;
 		return weak_match;
 	}
 	else {
+		if (matched_ref)
 			*matched_ref = matched;
 		return match;
 	}
@@ -1055,18 +1057,25 @@ static struct ref *alloc_delete_ref(void)
 	return ref;
 }
 
-static struct ref *try_explicit_object_name(const char *name)
+static int try_explicit_object_name(const char *name,
+				    struct ref **match)
 {
 	unsigned char sha1[20];
-	struct ref *ref;
 
-	if (!*name)
-		return alloc_delete_ref();
+	if (!*name) {
+		if (match)
+			*match = alloc_delete_ref();
+		return 0;
+	}
+
 	if (get_sha1(name, sha1))
-		return NULL;
-	ref = alloc_ref(name);
-	hashcpy(ref->new_sha1, sha1);
-	return ref;
+		return -1;
+
+	if (match) {
+		*match = alloc_ref(name);
+		hashcpy((*match)->new_sha1, sha1);
+	}
+	return 0;
 }
 
 static struct ref *make_linked_ref(const char *name, struct ref ***tail)
@@ -1103,6 +1112,7 @@ static int match_explicit_lhs(struct ref *src,
 {
 	switch (count_refspec_match(rs->src, src, match)) {
 	case 1:
+		if (allocated_match)
 			*allocated_match = 0;
 		return 0;
 	case 0:
@@ -1110,9 +1120,9 @@ static int match_explicit_lhs(struct ref *src,
 		 * not a reference name.  :refs/other is a
 		 * way to delete 'other' ref at the remote end.
 		 */
-		*match = try_explicit_object_name(rs->src);
-		if (!*match)
+		if (try_explicit_object_name(rs->src, match) < 0)
 			return error("src refspec %s does not match any.", rs->src);
+		if (allocated_match)
 			*allocated_match = 1;
 		return 0;
 	default: