help: add reference to template help (issue3413)

There is currently no clear link between the help for log
and the help on templates.  The log option is --template
but the template help is 'help templating' or 'help templates'.
This patch makes 'hg help template' work and also adds a
note into the log help explaining where to find more info.

mercurial/commands.py

@@ -3941,6 +3941,9 b' def log(ui, repo, *pats, **opts):'
     See :hg:`help revisions` and :hg:`help revsets` for more about
     specifying revisions.
 
+    See :hg:`help templates` for more about pre-packaged styles and
+    specifying custom templates.
+
     Returns 0 on success.
     """
 

mercurial/help.py

@@ -64,7 +64,7 b' helptable = sorted(['
     (['fileset', 'filesets'], _("Specifying File Sets"), loaddoc('filesets')),
     (['diffs'], _('Diff Formats'), loaddoc('diffs')),
     (['merge-tools'], _('Merge Tools'), loaddoc('merge-tools')),
-    (['templating', 'templates'], _('Template Usage'),
+    (['templating', 'templates', 'template', 'style'], _('Template Usage'),
      loaddoc('templates')),
     (['urls'], _('URL Paths'), loaddoc('urls')),
     (["extensions"], _("Using Additional Features"), extshelp),