remote rm/prune: print a message when writing packed-refs fails

Until v2.1.0-rc0~22^2~11 (refs.c: add an err argument to
repack_without_refs, 2014-06-20), repack_without_refs forgot to
provide an error message when commit_packed_refs fails.  Even today,
it only provides a message for callers that pass a non-NULL err
parameter.  Internal callers in refs.c pass non-NULL err but
"git remote" does not.

That means that "git remote rm" and "git remote prune" can fail
without printing a message about why.  Fix them by passing in a
non-NULL err parameter and printing the returned message.

This is the last caller to a ref handling function passing err ==
NULL.  A later patch can drop support for err == NULL, avoiding such
problems in the future.

Signed-off-by: Ronnie Sahlberg <sahlberg@google.com>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Reviewed-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: rsahlberg

builtin/remote.c

@@ -749,13 +749,16 @@ static int mv(int argc, const char **argv)
 
 static int remove_branches(struct string_list *branches)
 {
+	struct strbuf err = STRBUF_INIT;
 	const char **branch_names;
 	int i, result = 0;
 
 	branch_names = xmalloc(branches->nr * sizeof(*branch_names));
 	for (i = 0; i < branches->nr; i++)
 		branch_names[i] = branches->items[i].string;
-	result |= repack_without_refs(branch_names, branches->nr, NULL);
+	if (repack_without_refs(branch_names, branches->nr, &err))
+		result |= error("%s", err.buf);
+	strbuf_release(&err);
 	free(branch_names);
 
 	for (i = 0; i < branches->nr; i++) {
@@ -1332,9 +1335,13 @@ static int prune_remote(const char *remote, int dry_run)
 		delete_refs = xmalloc(states.stale.nr * sizeof(*delete_refs));
 		for (i = 0; i < states.stale.nr; i++)
 			delete_refs[i] = states.stale.items[i].util;
-		if (!dry_run)
-			result |= repack_without_refs(delete_refs,
-						      states.stale.nr, NULL);
+		if (!dry_run) {
+			struct strbuf err = STRBUF_INIT;
+			if (repack_without_refs(delete_refs, states.stale.nr,
+						&err))
+				result |= error("%s", err.buf);
+			strbuf_release(&err);
+		}
 		free(delete_refs);
 	}