dirstate: fix rebuild; add a test
If rebuild calls invalidate, it will just queue a (re)read of the dirstate
file, while what we really want is to empty the current state.
0 files updated, 0 files merged, 0 files removed, 0 files unresolved
0 files updated, 0 files merged, 0 files removed, 0 files unresolved
adding b
0 files updated, 0 files merged, 0 files removed, 0 files unresolved
adding b