pull: Do allow executing deltas when mirroring into bare{,-user}

pull: Do allow executing deltas when mirroring into bare{,-user}

In https://github.com/ostreedev/ostree/pull/408 we fixed a
bug where we would crash when trying to execute deltas into
an archive repo (which isn't presently supported).

But that was overly aggressive - we obviously *can* execute deltas
when mirroring into a bare repo.  This should fix a regression with
the way flatpak uses mirroring to pull from a user repo into the
system.

Closes: #506
Approved by: alexlarsson