dirstate: call and cache os.getcwd() in constructor

I'm about to make scmutil.matchfiles() not pass the root and cwd paths
to match.exact(), since they no longer have any effect. That turned
out to have the surprising effect of making some tests
(test-rebase-scenario-global.t and test-removeemptydirs.t) crash when
the working directory was removed. The problem was that my patch
removed the call to repo.getcwd(), which caused the current working
directory to not be cached in the dirstate as early as it was
before. This patch fixes that by caching the current working directory
in the dirstate constructor.

Differential Revision: https://phab.mercurial-scm.org/D5928

mercurial/dirstate.py

@@ -81,6 +81,10 b' class dirstate(object):'
         self._origpl = None
         self._updatedfiles = set()
         self._mapcls = dirstatemap
+        # Access and cache cwd early, so we don't access it for the first time
+        # after a working-copy update caused it to not exist (accessing it then
+        # raises an exception).
+        self._cwd
 
     @contextlib.contextmanager
     def parentchange(self):