unamend: error out when running on merge commit

It's better to error out than to produce a bad commit. We do that same
in `hg uncommit`. I haven't looked into how much work it is to make
them work instead.

Differential Revision: https://phab.mercurial-scm.org/D12087

hgext/uncommit.py

@@ -273,6 +273,8 b' def unamend(ui, repo, **opts):'
         curctx = repo[b'.']
 
         rewriteutil.precheck(repo, [curctx.rev()], b'unamend')
+        if len(curctx.parents()) > 1:
+            raise error.InputError(_(b"cannot unamend merge changeset"))
 
         # identify the commit to which to unamend
         markers = list(predecessormarkers(curctx))

tests/test-unamend.t

@@ -443,8 +443,5 b' Try to unamend a merge'
   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 !)
+  abort: cannot unamend merge changeset
+  [10]