obsstore: move _version to a propertycache

This makes sure _version is correct even if "_all" is not called.

mercurial/obsolete.py

@@ -555,7 +555,7 b' class obsstore(object):'
         # caches for various obsolescence related cache
         self.caches = {}
         self.svfs = svfs
-        self._version = defaultformat
+        self._defaultformat = defaultformat
         self._readonly = readonly
 
     def __iter__(self):
@@ -670,6 +670,13 b' class obsstore(object):'
         return self.svfs.tryread('obsstore')
 
     @propertycache
+    def _version(self):
+        if len(self._data) >= 1:
+            return _readmarkerversion(self._data)
+        else:
+            return self._defaultformat
+
+    @propertycache
     def _all(self):
         data = self._data
         if not data: