scmutil: introduce filecache...
scmutil: introduce filecache
The idea is being able to associate a file with a property, and watch
that file stat info for modifications when we decide it's important for it to
be up-to-date. Once it changes, we recreate the object.
On filesystems that can't uniquely identify a file, we always recreate.
As a consequence, localrepo.invalidate() will become much less expensive in the
case where nothing changed on-disk.
