Summary:
It turns out tracking ignored files does have an impact on status performance.
Filtering out the ignored directories is not that fast, and ignored files can
be scattered everywhere (like ".pyc", ".iml" files) that makes them harder to
be filtered out efficiently.
Add code paths to migrate between "ignored tracked" and "ignore untracked".
Store the metadata in treestate.
Reviewed By: phillco
Differential Revision: D12916021
fbshipit-source-id: e02d0c6f3b1a036f70703c11f35381c594e2f8e5
