fetch --prune: exit with error if pruning fails

When pruning refs fails, we print an error to stderr, but still
exit 0 from 'git fetch'.  Since this is a genuine error, fetch
should be exiting with some non-zero exit code.  Make it so.

The --prune option was introduced in f360d844de ("builtin-fetch: add
--prune option", 2009-11-10).  Unfortunately it's unclear from that
commit whether ignoring the exit code was an oversight or
intentional, but it feels like an oversight.

Helped-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Thomas Gummerer <t.gummerer@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/fetch.c

@@ -1609,12 +1609,14 @@ static int do_fetch(struct transport *transport,
 		 * don't care whether --tags was specified.
 		 */
 		if (rs->nr) {
-			prune_refs(rs, ref_map, transport->url);
+			retcode = prune_refs(rs, ref_map, transport->url);
 		} else {
-			prune_refs(&transport->remote->fetch,
-				   ref_map,
-				   transport->url);
+			retcode = prune_refs(&transport->remote->fetch,
+					     ref_map,
+					     transport->url);
 		}
+		if (retcode != 0)
+			retcode = 1;
 	}
 	if (fetch_and_consume_refs(transport, ref_map, worktrees)) {
 		free_refs(ref_map);

t/t5510-fetch.sh

@@ -164,6 +164,17 @@ test_expect_success 'fetch --prune --tags with refspec prunes based on refspec'
 	git rev-parse sometag
 '
 
+test_expect_success REFFILES 'fetch --prune fails to delete branches' '
+	cd "$D" &&
+	git clone . prune-fail &&
+	cd prune-fail &&
+	git update-ref refs/remotes/origin/extrabranch main &&
+	: this will prevent --prune from locking packed-refs for deleting refs, but adding loose refs still succeeds  &&
+	>.git/packed-refs.new &&
+
+	test_must_fail git fetch --prune origin
+'
+
 test_expect_success 'fetch --atomic works with a single branch' '
 	test_when_finished "rm -rf \"$D\"/atomic" &&