Copying refs from the system repo into a repo on a USB drive requires
the summary in the system repo to be up to date (and similarly for other
flatpak installations like a per-user one). At the moment we expect the
user to run `sudo ostree summary -u` before `flatpak create-usb` which
is a bad user experience. Another option is to set
`core.auto-update-summary` to true on the ostree repo config, but there
are significant performance concerns with that: it involves updating the
summary after every transaction rather than only when we need it. So
this commit changes the create-usb command to use the "UpdateSummary"
system-helper method to update the summary in the source repo before
copying to the destination. This strategy allows us to continue to let
non-root users use `flatpak create-usb`. This commit also tries to
update the remote repo metadata and appstream data for each remote
before copying to the USB, because we can now do that without
invalidating the summary.
Closes: #1945
Approved by: alexlarsson
1e75e645f1