dirstate: add identity information to detect simultaneous changing in storage

This identity is used to examine whether dirstate is simultaneously
changed in storage after previous caching (see issue5584 for detail).

util.cachestat can't be used for this purpose, because it has no
valuable information on Windows.

On the other hand, util.filestat can detect changing dirstate in
storage certainly, regardless of platforms.

    https://www.mercurial-scm.org/wiki/ExactCacheValidationPlan

Strictly speaking, if underlying filesystem doesn't support
ctime/mtime, util.filestat can't detect simultaneous changing in
storage as expected. But simultaneous changing on such (very rare)
platform can't be detected regardless of this patch series.

Therefore, util.filestat should be reasonable identity for almost all
usecases.

mercurial/dirstate.py

@@ -159,6 +159,11 b' class dirstate(object):'
         return self._copymap
 
     @propertycache
+    def _identity(self):
+        self._read()
+        return self._identity
+
+    @propertycache
     def _nonnormalset(self):
         nonnorm, otherparents = nonnormalentries(self._map)
         self._otherparentset = otherparents
@@ -426,6 +431,8 b' class dirstate(object):'
     def _read(self):
         self._map = {}
         self._copymap = {}
+        # ignore HG_PENDING because identity is used only for writing
+        self._identity = util.filestat(self._opener.join(self._filename))
         try:
             fp = self._opendirstatefile()
             try:
@@ -476,7 +483,8 b' class dirstate(object):'
         rereads the dirstate. Use localrepo.invalidatedirstate() if you want to
         check whether the dirstate has changed before rereading it.'''
 
-        for a in ("_map", "_copymap", "_filefoldmap", "_dirfoldmap", "_branch",
+        for a in ("_map", "_copymap", "_identity",
+                  "_filefoldmap", "_dirfoldmap", "_branch",
                   "_pl", "_dirs", "_ignore", "_nonnormalset",
                   "_otherparentset"):
             if a in self.__dict__:
@@ -741,6 +749,14 b' class dirstate(object):'
 
         self._dirty = True
 
+    def identity(self):
+        '''Return identity of dirstate itself to detect changing in storage
+
+        If identity of previous dirstate is equal to this, writing
+        changes based on the former dirstate out can keep consistency.
+        '''
+        return self._identity
+
     def write(self, tr):
         if not self._dirty:
             return