mercurial/obsolete.py

--- a/mercurial/obsolete.py	Thu Oct 09 00:15:04 2014 -0700
+++ b/mercurial/obsolete.py	Fri Oct 10 16:43:04 2014 -0500
@@ -112,6 +112,7 @@
 # bumped version and fix the situation, breaking the transitivity of
 # "bumped" here.
 bumpedfix = 1
+usingsha256 = 2
 
 ## Parsing and writing of version "0"
 #
@@ -193,6 +194,8 @@
 
 def _fm0encodeonemarker(marker):
     pre, sucs, flags, metadata, date, parents = marker
+    if flags & usingsha256:
+        raise util.Abort(_('cannot handle sha256 with old obsstore format'))
     metadata = dict(metadata)
     metadata['date'] = '%d %i' % date
     if parents is not None: