lib/repo: For bare-user, mask content object modes with 0775

lib/repo: For bare-user, mask content object modes with 0775

Having every object in a bare-user repo (and checkouts) be executable
is ugly.  I can't think of a good reason to do that; they should only
be executable if their input is.  This does
for `bare-user` what we did for `bare-user-only` in
https://github.com/ostreedev/ostree/pull/909
It's also a stronger version of what we do with `checkout -U` in suppressing
suid - here we also strip world-writable files and the sticky bit (even though
that's meaningless today, it might not be in the future).

Closes: https://github.com/ostreedev/ostree/issues/907
Closes: #908
Approved by: alexlarsson