obsstore: separate marker parsing from obsstore reading

This allows us to get raw obsstore content without parsing any markers.
Reading obsstore is much cheaper than parsing markers.

mercurial/obsolete.py

@@ -666,8 +666,12 b' class obsstore(object):'
         return self.add(transaction, markers)
 
     @propertycache
+    def _data(self):
+        return self.svfs.tryread('obsstore')
+
+    @propertycache
     def _all(self):
-        data = self.svfs.tryread('obsstore')
+        data = self._data
         if not data:
             return []
         self._version, markers = _readmarkers(data)