commit.c: free() revs.commit in get_fork_point()

Fix a memory leak that's been with us since d96855ff51 (merge-base:
teach "--fork-point" mode, 2013-10-23).

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

commit.c

@@ -1033,6 +1033,7 @@ struct commit *get_fork_point(const char *refname, struct commit *commit)
 	ret = bases->item;
 
 cleanup_return:
+	free(revs.commit);
 	free_commit_list(bases);
 	free(full_refname);
 	return ret;