hgweb: add index template to json/map

This template allows showing the list of all repos in an hgweb instance (in
hgwebdir mode) as json.

The test has "lastchange" globbed because hgweb uses here file modification
time and not the last commit time.

mercurial/templates/json/map

@@ -172,3 +172,12 b" help = '\\{"
   }'
 filenodelink = ''
 filenolink = ''
+index = '\{
+  "entries": [{join(entries%indexentry, ", ")}]
+  }'
+indexentry = '\{
+  "name": {name|utf8|json},
+  "description": {description|utf8|json},
+  "contact": {contact|utf8|json},
+  "lastchange": {lastchange|json}
+  }'

tests/test-hgwebdir.t

@@ -100,6 +100,23 b' should succeed'
   /a/
   /b/
   
+  $ get-with-headers.py localhost:$HGPORT '?style=json'
+  200 Script output follows
+  
+  {
+  "entries": [{
+  "name": "a",
+  "description": "unknown",
+  "contact": "Foo Bar \[e-mail]\u003e",
+  "lastchange": [*, *] (glob)
+  }, {
+  "name": "b",
+  "description": "unknown",
+  "contact": "Foo Bar \[e-mail]\u003e",
+  "lastchange": [*, *] (glob)
+  }]
+  } (no-eol)
+
   $ get-with-headers.py localhost:$HGPORT 'a/file/tip/a?style=raw'
   200 Script output follows