--- a/mercurial/context.py Fri Apr 14 12:34:26 2017 -0700
+++ b/mercurial/context.py Fri Apr 14 14:09:26 2017 +0200
@@ -1226,6 +1226,12 @@
continue
inrangep, linerange1 = _changesrange(c, p, linerange2, diffopts)
inrange = inrange or inrangep
+ # If revision 'i' has been seen (it's a merge), we assume that its
+ # line range is the same independently of which parents was used
+ # to compute it.
+ assert i not in seen or seen[i][1] == linerange1, (
+ 'computed line range for %s is not consistent between '
+ 'ancestor branches' % c)
seen[i] = c, linerange1
if inrange:
yield c, linerange1