builtin/history: unuse the commit buffer after use

Every call to repo_logmsg_reencode() must be paired with a call to
repo_unuse_commit_buffer(), or we may leak an allocated buffer. We
have such a leak in "git history", which we can fix by adding an unuse
call.

The leak-checking tests don't detect this because we only allocate a
fresh buffer sometimes: when the message is reencoded, or when we had
to load it fresh from the odb (e.g., because the commit was parsed
from the commit graph rather than the object contents). But you can
see it by running:

  make SANITIZE=leak
  cd t
  GIT_TEST_COMMIT_GRAPH=1 ./t3451-history-reword.sh

Helped-by: Jeff King <peff@peff.net>
Signed-off-by: Kaartic Sivaraam <kaartic.sivaraam@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/history.c

@@ -161,6 +161,7 @@ out:
 	free_commit_extra_headers(original_extra_headers);
 	strbuf_release(&commit_message);
 	free(original_author);
+	repo_unuse_commit_buffer(repo, commit_with_message, original_message);
 	return ret;
 }