manifest: make manifest.intersectfiles() internal
manifest.intersectfiles() is just a utility used by manifest.matches(), and
a future commit removes intersectfiles for treemanifest for optimization
purposes.
This commit makes the intersectfiles methods on manifestdict and treemanifest
internal, and converts its test to a more generic testMatches(), which has the
exact same coverage.
$ echo "[extensions]" >> $HGRCPATH
$ echo "mq=" >> $HGRCPATH
$ hg init
$ echo 'base' > base
$ hg ci -Ambase
adding base
$ hg qnew -mmqbase mqbase
$ hg qsave
$ hg qrestore 2
restoring status: hg patches saved state