treemanifest: make treemanifest.matches() faster
By converting treemanifest.matches() into a recursively additivie operation,
it becomes O(n).
The old matches function made a copy of the entire manifest and deleted
files that didn't match. With tree manifests, this was an O(n log n) operation
because del() was O(log n).
This change speeds up the command
"hg status --rev .^ 'relglob:*.js'
on the Mozilla repo, now taking 2.53s, down from 3.51s.