revset: add partial support for ancestor(wdir())
It's easy, so let's make it happen. I'm not certain if 'wdir() &' should
be required. ancestors(wdir()) works without it, but ancestor(wdir()) doesn't
as of now. That's the issue of fullreposet.__contains__() vs __and__().
#require docutils gettext
$ $TESTDIR/check-gendoc sv
checking for parse errors