When mirroring, write content directly, do not verify

When doing a pull --mirror from an archive-z2 repository into another
archive-z2 repository, currently we gunzip/checksum/gzip each content
object.  The re-gzip process in particular is fairly expensive.

This does assume that the upstream content is trusted and correct.
It'd be nice in the future to do at least a CRC check, if not the full
checksum.  (Could we append CRC data to the end of filez objects?)

We could also choose to only do this optimization if fetching over
TLS.

before: 1626 metadata, 20320 content objects fetched; 299634 KiB transferred in 62 seconds
after : 1626 metadata, 20320 content objects fetched; 299634 KiB transferred in 11 seconds

tests/test-pull-mirror-summary.sh

@@ -38,7 +38,7 @@ ostree --repo=${test_tmpdir}/ostree-srv/gnomerepo summary -u
 
 cd ${test_tmpdir}
 mkdir repo
-ostree --repo=repo init
+ostree --repo=repo init --mode=archive-z2
 ostree --repo=repo remote add --set=gpg-verify=false origin $(cat httpd-address)/ostree/gnomerepo
 ostree --repo=repo pull --mirror origin
 ostree --repo=repo checkout -U main main-copy