When we changed basematcher.visitdir() in 0ca205268beb (match: make
base matcher return True for visitdir, 2017-07-14), we forgot to add
an override in nevermatcher. This led to tests failing in narrowhg.
As Durham pointed out, it's high time to add unit tests for the
matcher, so this patch also adds a first unit test.
Differential Revision: https://phab.mercurial-scm.org/D151
