histedit: report when revisions to edit are not ancestors of working dir

Editing the history only is possible when the working dir is a descendant of
the revisions to edit. When this happens explain it by writing
  abort: %s is not an ancestor of working directory

hgext/histedit.py

@@ -498,8 +498,8 def histedit(ui, repo, *parent, **opts):
         keep = opts.get('keep', False)
         revs = between(repo, parent, topmost, keep)
         if not revs:
-            ui.warn(_('nothing to edit\n'))
-            return 1
+            raise util.Abort(_('%s is not an ancestor of working directory') %
+                             node.short(parent))
 
         ctxs = [repo[r] for r in revs]
         rules = opts.get('commands', '')

tests/test-histedit-revspec.t

@@ -65,5 +65,5 Run on a revision not ancestors of the c
   $ hg up 2
   1 files updated, 0 files merged, 0 files removed, 0 files unresolved
   $ hg histedit -r 4
-  nothing to edit
-  [1]
+  abort: 08d98a8350f3 is not an ancestor of working directory
+  [255]