histedit: use better meta-variable names than VALUE in help text

Before this change hg help histedit would use the default variable label:

    --commands VALUE
...
 -r --rev VALUE [+]

With this change the text will be in the usual help text style and a bit more
explanatory:

    --commands FILE
...
 -r --rev REV [+]

hgext/histedit.py

@@ -514,7 +514,7 b" actiontable = {'p': pick,"
 
 @command('histedit',
     [('', 'commands', '',
-      _('read history edits from the specified file')),
+      _('read history edits from the specified file'), _('FILE')),
      ('c', 'continue', False, _('continue an edit already in progress')),
      ('', 'edit-plan', False, _('edit remaining actions list')),
      ('k', 'keep', False,
@@ -523,7 +523,7 b" actiontable = {'p': pick,"
      ('o', 'outgoing', False, _('changesets not found in destination')),
      ('f', 'force', False,
       _('force outgoing even for unrelated repositories')),
-     ('r', 'rev', [], _('first revision to be edited'))],
+     ('r', 'rev', [], _('first revision to be edited'), _('REV'))],
      _("ANCESTOR | --outgoing [URL]"))
 def histedit(ui, repo, *freeargs, **opts):
     """interactively edit changeset history