bisect: improve English grammar of not-ancestors message

Multiple revisions cannot be a single ancestor.

Signed-off-by: Alex Henrie <alexhenrie24@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

bisect.c

@@ -747,7 +747,7 @@ static void handle_bad_merge_base(void)
 		exit(3);
 	}
 
-	fprintf(stderr, _("Some %s revs are not ancestor of the %s rev.\n"
+	fprintf(stderr, _("Some %s revs are not ancestors of the %s rev.\n"
 		"git bisect cannot work properly in this case.\n"
 		"Maybe you mistook %s and %s revs?\n"),
 		term_good, term_bad, term_good, term_bad);