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.
adding dir/bar
adding foo
dir/bar
foo
adding dir/bar_2
adding foo_2
dir/bar_2
foo_2
adding a
adding c
adding b
adding d
removing a
removing c
recording removal of a as rename to b (100% similar)