further fix traceback on invalid .hgignore patterns
Please apply this little fix (matchfn return None if no patterns are
specified).
#!/bin/sh
hg init
echo 123 > a
echo 123 > c
hg add a c
hg commit -m "first" -d "0 0" a c
echo 123 > b
hg status
echo 12 > c
hg status
hg add b
hg status
hg rm a
hg status
hg revert a
hg status
hg revert b
hg status
hg revert c
hg status
ls
true