help.merge-tools: do not double document merge tools

Merge tools were being double documented in help system output due
to functions being defined under multiple names in the merge tools
dictionary.

Establish a new dictionary for just the tools to document and
use it from the help system so we don't get double output.

Double documentation likely plagues other auto-documented items
as well. It might be a good idea to eventually compare function
instances to filter out duplicate entries from dictionaries
passed to ``makeitemsdoc``. However, without an easy way to break
ties, this may result in some functions being advertised over
their modern equivalents. This would be a noble patch series.
But it isn't one this author is willing to tackle at this time.

mercurial/filemerge.py

@@ -21,6 +21,8 b' def _toollist(ui, tool, part, default=[]'
     return ui.configlist("merge-tools", tool + "." + part, default)
 
 internals = {}
+# Merge tools to document.
+internalsdoc = {}
 
 def internaltool(name, trymerge, onfailure=None):
     '''return a decorator for populating internal merge tool table'''
@@ -29,6 +31,7 b' def internaltool(name, trymerge, onfailu'
         func.__doc__ = "``%s``\n" % fullname + func.__doc__.strip()
         internals[fullname] = func
         internals['internal:' + name] = func
+        internalsdoc[fullname] = func
         func.trymerge = trymerge
         func.onfailure = onfailure
         return func

mercurial/help.py

@@ -204,7 +204,8 b' def addtopicsymbols(topic, marker, symbo'
     addtopichook(topic, add)
 
 addtopicsymbols('filesets', '.. predicatesmarker', fileset.symbols)
-addtopicsymbols('merge-tools', '.. internaltoolsmarker', filemerge.internals)
+addtopicsymbols('merge-tools', '.. internaltoolsmarker',
+                filemerge.internalsdoc)
 addtopicsymbols('revsets', '.. predicatesmarker', revset.symbols)
 addtopicsymbols('templates', '.. keywordsmarker', templatekw.dockeywords)
 addtopicsymbols('templates', '.. filtersmarker', templatefilters.filters)