rebase: fix a memory leak

buf was never freed.

Signed-off-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/rebase.c

@@ -2165,6 +2165,7 @@ run_rebase:
 	ret = !!run_specific_rebase(&options, action);
 
 cleanup:
+	strbuf_release(&buf);
 	strbuf_release(&revisions);
 	free(options.head_name);
 	free(options.gpg_sign_opt);