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

tests/pull-test.sh

@@ -381,6 +381,12 @@ fi
 ${CMD_PREFIX} ostree --repo=repo fsck
 done
 
+# Test no-op with deltas: https://github.com/ostreedev/ostree/issues/1321
+cd ${test_tmpdir}
+repo_init --no-gpg-verify
+${CMD_PREFIX} ostree --repo=repo pull origin main
+${CMD_PREFIX} ostree --repo=repo pull --require-static-deltas origin main
+
 cd ${test_tmpdir}
 repo_init --no-gpg-verify
 ${CMD_PREFIX} ostree --repo=repo pull origin main@${prev_rev}