help: make "mergetool" an alias for "merge-tools"

I've probably typed `hg help mergetool` dozens of times. I'm tired
of it not working.

mercurial/help.py

@@ -214,7 +214,8 b' helptable = sorted(['
       _('Specifying Revisions'), loaddoc('revisions')),
     (['filesets', 'fileset'], _("Specifying File Sets"), loaddoc('filesets')),
     (['diffs'], _('Diff Formats'), loaddoc('diffs')),
-    (['merge-tools', 'mergetools'], _('Merge Tools'), loaddoc('merge-tools')),
+    (['merge-tools', 'mergetools', 'mergetool'], _('Merge Tools'),
+     loaddoc('merge-tools')),
     (['templating', 'templates', 'template', 'style'], _('Template Usage'),
      loaddoc('templates')),
     (['urls'], _('URL Paths'), loaddoc('urls')),