tests: demonstrate how `hg unamend` fails on merge commits

When `hg unamend` is run on a merge commit, it seems that it loses the
changes from the second parent.

Differential Revision: https://phab.mercurial-scm.org/D12086

tests/test-unamend.t

@@ -421,3 +421,30 b' Rename a->b, then amend b->c, and workin'
   A d
     b
   R b
+
+Try to unamend a merge
+
+  $ cd ..
+  $ hg init merge
+  $ cd merge
+  $ echo initial > initial
+  $ hg ci -Aqm initial
+  $ echo left > left
+  $ hg ci -Aqm left
+  $ hg co -q 0
+  $ echo right > right
+  $ hg ci -Aqm right
+  $ hg merge -q 1
+  $ hg ci -m merge
+  $ echo accidental > initial
+  $ hg st --rev 1 --rev .
+  A right
+  $ hg st --rev 2 --rev .
+  A left
+  $ hg amend
+  $ hg unamend
+  $ hg st --rev 1 --rev .
+  A right
+  R left (known-bad-output !)
+  $ hg st --rev 2 --rev .
+  A left (missing-correct-output !)