Before doing an update, flatpak fetches remote repo metadata to find
related refs and other information. But after fetching the metadata
flatpak fetches the remote summary to get the checksum that was just
fetched, which can fail if the computer is offline. So instead get the
checksum by reading the local repo, which has the side benefit of
eliminating a race condition (the summary being updated right after the
fetch happens).
Closes: #1397
Approved by: alexlarsson
e95d3763fe