pull: Optimize file:/// URIs to skip libsoup and hardlink if possible

It's always been suboptimal to have both pull and pull-local; as we go
beyond the raw object data into things like deltas and summary files,
the logic to perform e.g. mirroring should only be in one place.

This will be used by Pulp's OSTree content plugin at least to perform
promotions.

tests/pull-test.sh

@@ -55,6 +55,15 @@ ${CMD_PREFIX} ostree --repo=mirrorrepo fsck
 $OSTREE show main >/dev/null
 echo "ok pull mirror"
 
+cd ${test_tmpdir}
+mkdir mirrorrepo-local
+ostree --repo=mirrorrepo-local init --mode=archive-z2
+${CMD_PREFIX} ostree --repo=mirrorrepo-local remote add --set=gpg-verify=false origin file://$(pwd)/ostree-srv/gnomerepo
+${CMD_PREFIX} ostree --repo=mirrorrepo-local pull --mirror origin main
+${CMD_PREFIX} ostree --repo=mirrorrepo-local fsck
+$OSTREE show main >/dev/null
+echo "ok pull local mirror"
+
 cd ${test_tmpdir}
 ostree --repo=ostree-srv/gnomerepo commit -b main -s "Metadata string" --add-detached-metadata-string=SIGNATURE=HANCOCK --tree=ref=main
 ${CMD_PREFIX} ostree --repo=repo pull origin main