commit-reach(repo_get_merge_bases_many): pass on "missing commits" errors

The `merge_bases_many()` function was just taught to indicate parsing
errors, and now the `repo_get_merge_bases_many()` function is aware of
that, too.

Naturally, there are a lot of callers that need to be adjusted now, too.

Next stop: `repo_get_merge_bases_dirty()`.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

bisect.c

@@ -836,10 +836,11 @@ static void handle_skipped_merge_base(const struct object_id *mb)
 static enum bisect_error check_merge_bases(int rev_nr, struct commit **rev, int no_checkout)
 {
 	enum bisect_error res = BISECT_OK;
-	struct commit_list *result;
+	struct commit_list *result = NULL;
 
-	result = repo_get_merge_bases_many(the_repository, rev[0], rev_nr - 1,
-					   rev + 1);
+	if (repo_get_merge_bases_many(the_repository, rev[0], rev_nr - 1,
+				      rev + 1, &result) < 0)
+		exit(128);
 
 	for (; result; result = result->next) {
 		const struct object_id *mb = &result->item->object.oid;