commit: preserve extra when amending with commit --amend

The new extra propagation needs to be accounted for in cmdutil.amend,
when checking for a no-changes fast-path.

mercurial/cmdutil.py

@@ -2619,6 +2619,11 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,
@@ -2636,7 +2641,7 b' def amend(ui, repo, commitfunc, old, ext'
                 and newdesc == old.description()
                 and user == old.user()
                 and date == old.date()
-                and pureextra == old.extra()):
+                and pureextra == pureoldextra):
                 # nothing changed. continuing here would create a new node
                 # anyway because of the amend_source noise.
                 #

mercurial/commands.py

@@ -1617,6 +1617,9 b' def commit(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,