json: implement {help} template

We should consider add HTML rendering of the RST into the response as a
follow-up. I attempted to do this, but there was an empty array
returned by the rstdoc() template function. Not sure what's going on.
Will deal with it later.

mercurial/templates/json/map

@@ -92,6 +92,9 b" helptopicentry = '\\{"
   "topic": {topic|json},
   "summary": {summary|json}
   }'
-help = '"not yet implemented"'
+help = '\{
+  "topic": {topic|json},
+  "rawdoc": {doc|json}
+  }'
 filenodelink = ''
 filenolink = ''

tests/test-hgweb-json.t

@@ -1003,4 +1003,7 b' help/{topic} shows an individual help to'
   $ request json-help/phases
   200 Script output follows
   
-  "not yet implemented"
+  {
+    "rawdoc": "Working with Phases\n*", (glob)
+    "topic": "phases"
+  }