tests/test-issue433

match: add narrowmatcher class

This class can be used to adapt an existing match object to a new
match object that only cares about paths within a certain
subdirectory.

#!/bin/sh
# http://mercurial.selenic.com/bts/issue433

hg init a
cd a
echo a > a
hg commit -Ama

hg parents -r 0 doesnotexist
true