lib/commit: (refactor) Clean up delta bare write API

lib/commit: (refactor) Clean up delta bare write API

The way `_ostree_repo_open_content_bare()` did both looking for the object and
possibly creating a new fd was just weird and inconsistent with e.g. the pull
code where we always call `has_object()` first.

Just call `has_object()` in the delta paths that used this too, making the
implementation right now a thin wrapper around
`glnx_open_tmpfile_linkable_at()`, but this is prep for a later patch which does
more.

Closes: #1283
Approved by: jlebon