obsolete: have _readmarkers return the format version

readmarkers is not returning the version of the format it read from. This will
let callers know about the format and allow them to preserve it.

mercurial/obsolete.py

@@ -145,7 +145,7 b' def _readmarkers(data):'
     if diskversion not in formats:
         raise util.Abort(_('parsing obsolete marker: unknown version %r')
                          % diskversion)
-    return formats[diskversion][0](data, off)
+    return diskversion, formats[diskversion][0](data, off)
 
 def _fm0readmarkers(data, off=0):
     """Read and enumerate markers from raw data in format version 0"""
@@ -312,7 +312,8 b' class obsstore(object):'
         self.sopener = sopener
         data = sopener.tryread('obsstore')
         if data:
-            self._load(_readmarkers(data))
+            version, markers = _readmarkers(data)
+            self._load(markers)
 
     def __iter__(self):
         return iter(self._all)
@@ -396,7 +397,7 b' class obsstore(object):'
         """merge a binary stream of markers inside the obsstore
 
         Returns the number of new markers added."""
-        markers = _readmarkers(data)
+        version, markers = _readmarkers(data)
         return self.add(transaction, markers)
 
     def _load(self, markers):