amend: allow amend of non-head when obsolete is enabled

Obsolescence marker can represent this situation just fine. The old
version is marked as precursor of the new changeset. All its
descendants become "unstable".

If obsolescence is not enabled we keep the current behavior of
aborting. This new behavior only applies when obsolete is
enabled and is subject to future discussion and changes.

mercurial/commands.py

@@ -1296,7 +1296,7 b' def commit(ui, repo, *pats, **opts):'
             raise util.Abort(_('cannot amend merge changesets'))
         if len(repo[None].parents()) > 1:
             raise util.Abort(_('cannot amend while merging'))
-        if old.children():
+        if (not obsolete._enabled) and old.children():
             raise util.Abort(_('cannot amend changeset with children'))
 
         e = cmdutil.commiteditor

tests/test-commit-amend.t

@@ -458,3 +458,23 b' Test that amend does not make it easy to'
   $ hg commit --amend
   $ hg id
   b99e5df575f7 (a) tip
+
+Test ui.prevent-unstable
+---------------------------------------------------------------------
+
+  $ hg up '.^'
+  0 files updated, 0 files merged, 1 files removed, 0 files unresolved
+  $ echo 'b' >> a
+  $ hg log --style compact -r 'children(.)'
+  18[tip]:11   b99e5df575f7   1970-01-01 00:00 +0000   test
+    babar
+  
+  $ hg commit --amend
+  $ hg log -r 'unstable()'
+  changeset:   18:b99e5df575f7
+  branch:      a
+  parent:      11:3334b7925910
+  user:        test
+  date:        Thu Jan 01 00:00:00 1970 +0000
+  summary:     babar
+