rhg: remember the inode of .hg/dirstate...
rhg: remember the inode of .hg/dirstate
This allows us to detect changes of `.hg/dirstate`, which is either the
full dirstate (in dirstate-v1) or the docket file (v2) without relying on
data inside the file. It only works on UNIX systems.
This fixes a race condition for dirstate-v1 (as demonstrated by
the test changes) and adds a confortable layer of sanity for dirstate-v2.