histedit: always define update results

Before, we had a branch that could return None for the update stats.
Let's just return an updateresult instance instead.

Differential Revision: https://phab.mercurial-scm.org/D2693

hgext/histedit.py

@@ -566,7 +566,7 b' def applychanges(ui, repo, ctx, opts):'
         # edits are "in place" we do not need to make any merge,
         # just applies changes on parent for editing
         cmdutil.revert(ui, repo, ctx, (wcpar, node.nullid), all=True)
-        stats = None
+        stats = mergemod.updateresult(0, 0, 0, 0)
     else:
         try:
             # ui.forcemerge is an internal variable, do not document