histedit: edit with custom filename

For instance, Emacs allows to open file with special features enabled
(AKA mode) based on the path/name of the file [1]. For such cases, use
'hg-histedit-XXX.txt' as filename pattern.

[1] https://www.gnu.org/software/emacs/manual/html_node/emacs/Choosing-Modes.html

hgext/histedit.py

@@ -1126,7 +1126,7 b' def ruleeditor(repo, ui, rules, editcomm'
     rules = '\n'.join([makedesc(repo, act, rev) for [act, rev] in rules])
     rules += '\n\n'
     rules += editcomment
-    rules = ui.edit(rules, ui.username())
+    rules = ui.edit(rules, ui.username(), {'prefix': 'histedit'})
 
     # Save edit rules in .hg/histedit-last-edit.txt in case
     # the user needs to ask for help after something