dirstate: remove layering violation around writing dirstate out

This violation, which passes repo object to dirstate, was introduced
by 09bb1ee7e73e.

This patch uses 'False' instead of 'None' as default value of 'tr'
argument, to distinguish "None as repo.currenttransaction() result"
from "legacy invocation without explicit tr passing".

mercurial/dirstate.py

@@ -648,7 +648,7 b' class dirstate(object):'
         self._pl = (parent, nullid)
         self._dirty = True
 
-    def write(self, repo=None):
+    def write(self, tr=False):
         if not self._dirty:
             return
 
@@ -660,17 +660,13 b' class dirstate(object):'
             time.sleep(delaywrite)
 
         filename = self._filename
-        if not repo:
-            tr = None
+        if tr is False: # not explicitly specified
             if self._opener.lexists(self._pendingfilename):
                 # if pending file already exists, in-memory changes
                 # should be written into it, because it has priority
                 # to '.hg/dirstate' at reading under HG_PENDING mode
                 filename = self._pendingfilename
-        else:
-            tr = repo.currenttransaction()
-
-        if tr:
+        elif tr:
             # 'dirstate.write()' is not only for writing in-memory
             # changes out, but also for dropping ambiguous timestamp.
             # delayed writing re-raise "ambiguous timestamp issue".
@@ -678,7 +674,7 b' class dirstate(object):'
             # https://www.mercurial-scm.org/wiki/DirstateTransactionPlan
 
             # emulate dropping timestamp in 'parsers.pack_dirstate'
-            now = _getfsnow(repo.vfs)
+            now = _getfsnow(self._opener)
             dmap = self._map
             for f, e in dmap.iteritems():
                 if e[0] == 'n' and e[3] == now: