repo: Create metadata commit in ostree_repo_regenerate_metadata

repo: Create metadata commit in ostree_repo_regenerate_metadata

Rather than creating the `ostree-metadata` commit in the summary
builtin, do it in the new `ostree_repo_regenerate_metadata` API. The
commit contents are unchanged and the commit is generated before the
summary as before. To keep from triggering an extra summary update,
automatic summary updating is disabled in the transaction.

Since the summary builtin was already using the new API, it will
continue to generate the `ostree-metadata` commit when the repo has a
collection ID. However, the `ostree_repo_regenerate_summary` API will
still only generate the summary file as before.