match: override 'visitdir' in subdirmatcher
The manifest.manifest class has a _treeinmem member than one can
manually set to True to test that the treemanifest class works as a
drop-in replacement for manifestdict (which is mostly a requirement
for treemanifest repos to work). However, it doesn't quite work at the
moment. These tests fail:
test-largefiles-misc.t
test-rebase-newancestor.t
test-subrepo.t
test-subrepo-deep-nested-change.t
test-subrepo-recursion.t
All but test-rebase-newancestor.t fail because they trigger calls to
subdirmatcher.visitdir(), which tries to access a _excluderoots field
that does not exist on the subdirmatcher. Let's fix that by overriding
visitdir() in a similar way to how matchfn is overridden, i.e. by
prepending the directory before calling the superclass method.