MINOR: pattern: add pat_ref_commit() to commit a previously inserted element

MINOR: pattern: add pat_ref_commit() to commit a previously inserted element

This function will be used after a successful pat_ref_append() to propagate
the pattern to all use places (including parsing and indexing). On failure,
it will entirely roll back all insertions and free the pattern itself. It
also preserves the generation number so that it is convenient for use in
association with pat_ref_append(). pat_ref_add() was modified to rely on
it instead of open-coding the insertion and roll-back.