dirstate: factor the identity setting code in the dirstate map

We need it in more locations, so let us start factoring thing out first
to make sure the same code is called everywhere.

This bears some similarity with 85746485a4dd on default, but at a smaller
scope and for a different purpose.

mercurial/dirstatemap.py

@@ -76,6 +76,11 b' class _dirstatemapcommon:'
         # for consistent view between _pl() and _read() invocations
         self._pendingmode = None
 
+    def _set_identity(self):
+        # ignore HG_PENDING because identity is used only for writing
+        file_path = self._opener.join(self._filename)
+        self.identity = util.filestat.frompath(file_path)
+
     def preload(self):
         """Loads the underlying data, if it's not already loaded"""
         self._map
@@ -295,9 +300,7 b' class dirstatemap(_dirstatemapcommon):'
 
     def read(self):
         # ignore HG_PENDING because identity is used only for writing
-        self.identity = util.filestat.frompath(
-            self._opener.join(self._filename)
-        )
+        self._set_identity()
 
         testing.wait_on_cfg(self._ui, b'dirstate.pre-read-file')
         if self._use_dirstate_v2:
@@ -563,9 +566,7 b' if rustmod is not None:'
             Fills the Dirstatemap when called.
             """
             # ignore HG_PENDING because identity is used only for writing
-            self.identity = util.filestat.frompath(
-                self._opener.join(self._filename)
-            )
+            self._set_identity()
 
             testing.wait_on_cfg(self._ui, b'dirstate.pre-read-file')
             if self._use_dirstate_v2: