status: invalidate dirstate on LockError

If we cannot take the lock, someone else is modifying the repository. Let us
discard dirstate uncommitted data before exiting the status code.

Having a clean dirstate after such operation seems safer.

Strictly speaking, there is a small behavior change in the following situation:

* process A call `status` outside of the `wlock`
* process B grab the `wlock`
* process A fails to acquires the lock to write status fixup
* process B release the `wlock` *without touching the dirstate*
* process A later grab the `wlock`
* process A can write dirstate update from earlier `status`

However this is a fairly hypothetical situation :
* process A has to be raced
* process B have to not update the dirstate
* process A has to run another *unrelated* operation later.

This seems rare enough to overlook.

I am stating that the two operations in process A has to be unrelated.
Otherwise,  collecting status data outside of the lock to use them inside the
lock is racy. Any other process could move things around (eg: the working copy)
making the data collected during status irrelevantor even harmful.

If such code exists, it should be fixed ASAP.

mercurial/context.py

@@ -1889,7 +1889,7 b' class workingctx(committablectx):'
                         for ps in poststatus:
                             ps(self, status)
             except error.LockError:
-                pass
+                dirstate.invalidate()
             finally:
                 # Even if the wlock couldn't be grabbed, clear out the list.
                 self._repo.clearpostdsstatus()