This makes the ostree trivial-httpd --autoexit feature work better,
because it seems to exit whenever the root directory changes (i.e. not
only when its deleted).
This means the root dir can't be the repo (because then we can't
update the repo), or the base testdir (because we create files there
too), so instead we make the repo $testdir/repos/test and
$testdir/repos as the httpd root.
If the bundle contains an origin link we can now install related
things from it, such as locale data.
You can also build the bundle with --runtime-repo=URL, where the url
points to a flatpakrepo file for a repo with runtimes. This works
similar to the RuntimeRepo= feature in flatpakref files.
