dir: Fix P2P build failure

Apparently when I rebased https://github.com/flatpak/flatpak/pull/1314
to master, git cleanly applied the patch in a different place than it
was originally. Commit c5ab9e22b moved the find_remotes_async() call to
find_latest_rev(), so all that needs to be done is pass along the
checksum in check_for_update(). This way when you're trying to downgrade
something the specified commit will be searched for instead of the
latest one.

Fixes https://github.com/flatpak/flatpak/issues/1431

Closes: #1435
Approved by: barthalion
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