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 a
adding b
copy .hg/patches to .hg/patches.1
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
M b
a
b
merging with queue at: .hg/patches.1
applying rm_a
Now at: rm_a
b
Patch queue now empty