static-delta: Pretend that world unreadable objects are new objects

static-delta: Pretend that world unreadable objects are new objects

This will prevent including in the delta the bits to update files that
are not world readable, so that we don't run into a permissions problem
when applying the deltas from a bare-user repository that has a bare
repository set as its parent.

This is the case for Endless when updating flatpak runtimes, as the
temporary directory created in ~/.local/share/flatpak/system-cache will
be of type bare-user with its parent set to /var/lib/flatpak which is a
bare repository in EOS, as it's shared with the one at /ostree/repo.

https://phabricator.endlessm.com/T14159

Closes: #634
Approved by: cgwalters