scmutil: update cached copy when filecached attribute is assigned (issue3263)
When assigning a new object to filecached properties, the cached object that
was kept in the _filecache map was still holding the old object.
By implementing __set__, we track these changes too and update the cached
copy as well.
cmdserver: invalidate the dirstate when running commands (issue3271)
The dirstate is invalidated separately outside of invalidate() which is
already being called (other callers of invalidate() seems to suggest the
separation is there for a reason).