changelog: don't use generaldelta

mercurial/changelog.py

@@ -100,6 +100,10 b' def delayopener(opener, target, divert, '
 class changelog(revlog.revlog):
     def __init__(self, opener):
         revlog.revlog.__init__(self, opener, "00changelog.i")
+        if self._initempty:
+            # changelogs don't benefit from generaldelta
+            self.version &= ~revlog.REVLOGGENERALDELTA
+            self._generaldelta = False
         self._realopener = opener
         self._delayed = False
         self._divert = False

mercurial/revlog.py

@@ -234,12 +234,14 b' class revlog(object):'
                 v = 0
 
         i = ''
+        self._initempty = True
         try:
             f = self.opener(self.indexfile)
             i = f.read()
             f.close()
             if len(i) > 0:
                 v = struct.unpack(versionformat, i[:4])[0]
+                self._initempty = False
         except IOError, inst:
             if inst.errno != errno.ENOENT:
                 raise