help: drop help for branches template keyword

The {branches} keyword dates to pre-1.0 Mercurial's tag-like branch
scheme which allowed changesets to be on multiple branches. This is
the last visible vestige of that scheme, users should instead be using
{branch}, possibly with if().

mercurial/templatekw.py

@@ -390,6 +390,7 b' dockeywords = {'
     'parents': _showparents,
 }
 dockeywords.update(keywords)
+del dockeywords['branches']
 
 # tell hggettext to extract docstrings from these functions:
 i18nfunctions = dockeywords.values()