dirstate: avoid reading the map when possible (issue5713) (issue5717)

Before the recent refactor, we would not load the entire map until it was
accessed. As part of the refactor, that got lost and even just trying to load
the dirstate parents would load the whole map. This caused a perf regression
(issue5713) and a regression with static http serving (issue5717).

Making it lazy loaded again fixes both.

Differential Revision: https://phab.mercurial-scm.org/D1253

mercurial/dirstate.py

@@ -129,7 +129,7 b' class dirstate(object):'
     def _map(self):
         '''Return the dirstate contents as a map from filename to
         (state, mode, size, time).'''
-        self._read()
+        self._map = dirstatemap(self._ui, self._opener, self._root)
         return self._map
 
     @property
@@ -353,10 +353,6 b' class dirstate(object):'
             f.discard()
             raise
 
-    def _read(self):
-        self._map = dirstatemap(self._ui, self._opener, self._root)
-        self._map.read()
-
     def invalidate(self):
         '''Causes the next access to reread the dirstate.
 
@@ -1201,14 +1197,24 b' class dirstatemap(object):'
         self._root = root
         self._filename = 'dirstate'
 
-        self._map = {}
-        self.copymap = {}
         self._parents = None
         self._dirtyparents = False
 
         # for consistent view between _pl() and _read() invocations
         self._pendingmode = None
 
+    @propertycache
+    def _map(self):
+        self._map = {}
+        self.read()
+        return self._map
+
+    @propertycache
+    def copymap(self):
+        self.copymap = {}
+        self._map
+        return self.copymap
+
     def clear(self):
         self._map = {}
         self.copymap = {}
@@ -1388,7 +1394,7 b' class dirstatemap(object):'
 
     @propertycache
     def identity(self):
-        self.read()
+        self._map
         return self.identity
 
     @propertycache