extdiff: correctly handle deleted subrepositories (issue3153)
Previously, when extdiff was called on two changesets where
a subrepository had been removed, an unexpected KeyError would
be raised.
Now, the missing subrepository will be ignored. This behavior
mirrors the behavior in diffordiffstat from cmdutil.py line
~1138-1153. The KeyError is caught and the revision is
set to None.
try/catch of LookupError around matchmod.narrowmatcher and
sub.status is removed, as LookupError is not raised anywhere
within those methods or deeper calls.