context: add an assertion checking linerange consistency in blockdescendants()

If this assertion fails, this indicates a flaw in the algorithm. So fail fast
instead of possibly producing wrong results.

Also extend the target line range in test to catch a merge changeset with all
its parents.

mercurial/context.py

@@ -1226,6 +1226,12 b' def blockdescendants(fctx, fromline, tol'
                 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

tests/test-annotate.t

@@ -606,7 +606,7 b' we follow all branches in descending dir'
   $ mv baz.new baz
   $ hg ci -m 'baz:3->+3'
   created new head
-  $ hg log -T '{rev}: {desc}\n' -r 'followlines(baz, 3:5, startrev=16, descend=True)' --graph
+  $ hg log -T '{rev}: {desc}\n' -r 'followlines(baz, 2:5, startrev=16, descend=True)' --graph
   @  29: baz:3->+3
   :
   : o  26: baz:3+->3-
@@ -615,10 +615,11 b' we follow all branches in descending dir'
   :/
   o    20: baz:4
   |\
-  | ~
-  o  19: baz:3
-  |
-  ~
+  | o  19: baz:3
+  |/
+  o    18: baz:2
+  |\
+  ~ ~
 
 check error cases
   $ hg up 23 --quiet