json: implement {tags} template

Tags is pretty easy to implement. Let's start there.

The output is slightly different from `hg tags -Tjson`. For reference,
the CLI has the following output:

  [
   {
    "node": "e2049974f9a23176c2addb61d8f5b86e0d620490",
    "rev": 29880,
    "tag": "tip",
    "type": ""
   },
   ...
  ]

Our output has the format:

  {
    "node": "0aeb19ea57a6d223bacddda3871cb78f24b06510",
    "tags": [
      {
        "node": "e2049974f9a23176c2addb61d8f5b86e0d620490",
        "tag": "tag1",
        "date": [1427775457.0, 25200]
      },
      ...
    ]
  }

"rev" is omitted because it isn't a reliable identifier. We shouldn't
be exposing them in web APIs and giving the impression it remotely
resembles a stable identifier. Perhaps we could one day hide this behind
a config option (it might be useful to expose when running servers
locally).

The "type" of the tag isn't defined because this information isn't yet
exposed to the hgweb templater (it could be in a follow-up) and because
it is questionable whether different types should be exposed at all.
(Should the web interface really be exposing "local" tags?)

We use an object for the outer type instead of Array for a few reasons.
First, it is extensible. If we ever need to throw more global properties
into the output, we can do that without breaking backwards compatibility
(property additions should be backwards compatible). Second, uniformity
in web APIs is nice. Having everything return objects seems much saner than
a mix of array and object. Third, there are security issues with arrays
in older browsers. The JSON web services world almost never uses arrays
as the main type for this reason.

Another possibly controversial part about this patch is how dates are
defined. While JSON has a Date type, it is based on the JavaScript Date
type, which is widely considered a pile of garbage. It is a non-starter
for this reason.

Many of Mercurial's built-in date filters drop seconds resolution. So
that's a non-starter as well, since we want the API to be lossless where
possible. rfc3339date, rfc822date, isodatesec, and date are all lossless.
However, they each require the client to perform string parsing on top of
JSON decoding. While date parsing libraries are pretty ubiquitous, some
languages don't have them out of the box. However, pretty much every
programming language can deal with UNIX timestamps (which are just
integers or floats). So, we choose to use Mercurial's internal date
representation, which in JSON is modeled as float seconds since UNIX
epoch and an integer timezone offset from UTC (keep in mind
JavaScript/JSON models all "Numbers" as double prevision floating point
numbers, so there isn't a difference between ints and floats in JSON).

mercurial/templates/json/map

@@ -5,7 +5,15 b' shortlog = \'"not yet implemented"\''
 changelog = '"not yet implemented"'
 changeset = '"not yet implemented"'
 manifest = '"not yet implemented"'
-tags = '"not yet implemented"'
+tags = '\{
+  "node": {node|json},
+  "tags": [{join(entriesnotip%tagentry, ", ")}]
+  }'
+tagentry = '\{
+  "tag": {tag|json},
+  "node": {node|json},
+  "date": {date|json}
+  }'
 bookmarks = '"not yet implemented"'
 branches = '"not yet implemented"'
 summary = '"not yet implemented"'

tests/test-hgweb-json.t

@@ -164,7 +164,27 b' tags/ shows tags info'
   $ request json-tags
   200 Script output follows
   
-  "not yet implemented"
+  {
+    "node": "6ab967a8ab3489227a83f80e920faa039a71819f",
+    "tags": [
+      {
+        "date": [
+          0.0,
+          0
+        ],
+        "node": "f2890a05fea49bfaf9fb27ed5490894eba32da78",
+        "tag": "tag2"
+      },
+      {
+        "date": [
+          0.0,
+          0
+        ],
+        "node": "78896eb0e102174ce9278438a95e12543e4367a7",
+        "tag": "tag1"
+      }
+    ]
+  }
 
 bookmarks/ shows bookmarks info