lib/pull: Avoid error if current with --require-static-deltas

lib/pull: Avoid error if current with --require-static-deltas

A tricky thing here that caused this to go past a lot of our tests
is that the code was mostly OK if there was an available delta from
an older commit.  But this case broke if we e.g. had a new OS
deployment and did a `--require-static-deltas` pull, i.e. the initial
state.

I cleaned up our "find static delta state" function to return an enumeration,
and extended it with an "already have the commit" state.  A problem
I then hit is that we've historically fetched detached metadata for
non-delta pulls, even if the commit hasn't changed.  I decided not to
do that for `--require-static-deltas` pulls for now; otherwise the
code gets notably more complex.

Closes: https://github.com/ostreedev/ostree/issues/1321
Closes: #1323
Approved by: jlebon