introduce fncache repository layout
* adds a new entry 'fncache' to '.hg/requires' for new repos
* writes new file '.hg/store/fncache'
* hash-encodes filenames with long paths (
issue839)
* encodes Windows reserved filenames (
issue793)
Make it possible to disable filtering for a pattern.
Rationale: Apparently there were no way to disable for example win32text extension locally.
RFC: Can it already be done another way?