incoming: fix incoming when a local head is remotely filtered (issue3805)

In its current state discovery may return (remotely) filtered elements
in "common". This has usually no impact as "missing" is kept clear of
filtered elements. However when the "remote" repo is a local repo (disk
accessible, and directly created in memory) the incoming code takes a
shortcut and directly uses the "remote" repo to generate the incoming
output. When some common elements are filtered this led to a crash. We
now ensure we use an unfiltered repository to generate the incoming
output. This does not change the behavior as missing is clear of
filtered revision.

Now that we have proper low level filtering, incoming code needs a
deeper cleanup but it is already planned.

--HG--
branch : stable
extra : rebase_source : 386828df2e047ee07f322293f766744ac82b2626
extra : amend_source : bf44d8eb7440a3dd7de83de6668a4cb81354d999