amend: don't preserve most extra fields

This backs out changeset 39163708825cd.

There are some extra fields that absolutely should not be preserved, like the
convert_revision field introduced by the convert and hgsubversion extensions.
The problem with extensions blacklisting certain extra fields is that they
might not be enabled at the time the amend is performed.

In the long run we probably want separately marked transferable and
non-transferable extra fields, but for now restore the old Mercurial 3.6
behavior.

mercurial/cmdutil.py

@@ -2623,11 +2623,6 b' def amend(ui, repo, commitfunc, old, ext'
                 message = old.description()
 
             pureextra = extra.copy()
-            if 'amend_source' in pureextra:
-                del pureextra['amend_source']
-            pureoldextra = old.extra()
-            if 'amend_source' in pureoldextra:
-                del pureoldextra['amend_source']
             extra['amend_source'] = old.hex()
 
             new = context.memctx(repo,
@@ -2645,7 +2640,7 b' def amend(ui, repo, commitfunc, old, ext'
                 and newdesc == old.description()
                 and user == old.user()
                 and date == old.date()
-                and pureextra == pureoldextra):
+                and pureextra == old.extra()):
                 # nothing changed. continuing here would create a new node
                 # anyway because of the amend_source noise.
                 #

mercurial/commands.py

@@ -1682,9 +1682,6 b' def _docommit(ui, repo, *pats, **opts):'
         if not allowunstable and old.children():
             raise error.Abort(_('cannot amend changeset with children'))
 
-        newextra = extra.copy()
-        newextra['branch'] = branch
-        extra = newextra
         # commitfunc is used only for temporary amend commit by cmdutil.amend
         def commitfunc(ui, repo, message, match, opts):
             return repo.commit(message,