3. Note the added FIXME in `vc-default-working-revision', where I'm not
sure if it should be declared obsolete or not. See Stefan M's commit
6e5d0e9e73.
This function is never called directly (the only way it could be called
was via (vc-call-backend .. 'working-revision) for backends that
defined `vc-<backend>-workfile-revision` instead of
`vc-<backend>-working-revision`.
So a `make-obsolete` would have been ineffective (since it's never
called directly, the byte-compiler would never emit an obsolescence
warning) and I replaced it with a more annoying "message". Since noone
complained about that message, I think it's definitely safe to remove
the function.