lib/repo-commit: Abort a transaction if preparing it fails

lib/repo-commit: Abort a transaction if preparing it fails

If ostree_repo_prepare_transaction() fails, we should reset the
repository’s state so that the failed call was essentially idempotent.
Do that by calling ostree_repo_abort_transaction() on the failure path.

Typically, the way for preparing a transaction to fail is for its
GCancellable to be triggered, rather than because any of the operations
involved in preparing a transaction are particularly failure prone.

Signed-off-by: Philip Withnall <withnall@endlessm.com>
Closes: #1647
Approved by: cgwalters