dirstate: factor the identity getting/setting code in the dirstate map

We are doing the same things twice and we will add more logic in the next
changesets. So lets start factoring things out now.

mercurial/dirstatemap.py

@@ -58,6 +58,15 b' class _dirstatemapcommon:'
         # for consistent view between _pl() and _read() invocations
         self._pendingmode = None
 
+    def _set_identity(self):
+        self.identity = self._get_current_identity()
+
+    def _get_current_identity(self):
+        try:
+            return util.cachestat(self._opener.join(self._filename))
+        except FileNotFoundError:
+            return None
+
     def preload(self):
         """Loads the underlying data, if it's not already loaded"""
         self._map
@@ -263,10 +272,7 b' class dirstatemap(_dirstatemapcommon):'
 
     def read(self):
         # ignore HG_PENDING because identity is used only for writing
-        try:
-            self.identity = util.cachestat(self._opener.join(self._filename))
-        except FileNotFoundError:
-            self.identity = None
+        self._set_identity()
 
         if self._use_dirstate_v2:
             if not self.docket.uuid:
@@ -529,12 +535,7 b' if rustmod is not None:'
             Fills the Dirstatemap when called.
             """
             # ignore HG_PENDING because identity is used only for writing
-            try:
-                self.identity = util.cachestat(
-                    self._opener.join(self._filename)
-                )
-            except FileNotFoundError:
-                self.identity = None
+            self._set_identity()
 
             if self._use_dirstate_v2:
                 if self.docket.uuid: