store: refactor the fncache handling
put all the fncache file writing and reading in
the same place.
dirty
M foo
% validate update of standalone execute bit change
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
nothing changed
0 files updated, 0 files merged, 0 files removed, 0 files unresolved
0 files updated, 0 files merged, 0 files removed, 0 files unresolved