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

@@ -1,20 +1,28 b''
 mimetype = 'application/json'
 filerevision = '"not yet implemented"'
 search = '"not yet implemented"'
 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"'
 filediff = '"not yet implemented"'
 filecomparison = '"not yet implemented"'
 fileannotate = '"not yet implemented"'
 filelog = '"not yet implemented"'
 graph = '"not yet implemented"'
 helptopics = '"not yet implemented"'
 help = '"not yet implemented"'
 filenodelink = ''
 filenolink = ''

tests/test-hgweb-json.t

@@ -1,241 +1,261 b''
 #require json
 #require serve
 
   $ request() {
   >   $TESTDIR/get-with-headers.py --json localhost:$HGPORT "$1"
   > }
 
   $ hg init test
   $ cd test
   $ mkdir da
   $ echo foo > da/foo
   $ echo foo > foo
   $ hg -q ci -A -m initial
   $ echo bar > foo
   $ hg ci -m 'modify foo'
   $ echo bar > da/foo
   $ hg ci -m 'modify da/foo'
   $ hg bookmark bookmark1
   $ hg up default
   0 files updated, 0 files merged, 0 files removed, 0 files unresolved
   (leaving bookmark bookmark1)
   $ hg mv foo foo-new
   $ hg commit -m 'move foo'
   $ hg tag -m 'create tag' tag1
   $ echo baz > da/foo
   $ hg commit -m 'another commit to da/foo'
   $ hg tag -m 'create tag2' tag2
   $ hg bookmark bookmark2
   $ hg -q up -r 0
   $ hg -q branch test-branch
   $ echo branch > foo
   $ hg commit -m 'create test branch'
 
   $ hg log -G
   @  changeset:   7:6ab967a8ab34
   |  branch:      test-branch
   |  tag:         tip
   |  parent:      0:06e557f3edf6
   |  user:        test
   |  date:        Thu Jan 01 00:00:00 1970 +0000
   |  summary:     create test branch
   |
   | o  changeset:   6:ceed296fe500
   | |  bookmark:    bookmark2
   | |  user:        test
   | |  date:        Thu Jan 01 00:00:00 1970 +0000
   | |  summary:     create tag2
   | |
   | o  changeset:   5:f2890a05fea4
   | |  tag:         tag2
   | |  user:        test
   | |  date:        Thu Jan 01 00:00:00 1970 +0000
   | |  summary:     another commit to da/foo
   | |
   | o  changeset:   4:93a8ce14f891
   | |  user:        test
   | |  date:        Thu Jan 01 00:00:00 1970 +0000
   | |  summary:     create tag
   | |
   | o  changeset:   3:78896eb0e102
   | |  tag:         tag1
   | |  user:        test
   | |  date:        Thu Jan 01 00:00:00 1970 +0000
   | |  summary:     move foo
   | |
   | o  changeset:   2:8d7c456572ac
   | |  bookmark:    bookmark1
   | |  user:        test
   | |  date:        Thu Jan 01 00:00:00 1970 +0000
   | |  summary:     modify da/foo
   | |
   | o  changeset:   1:f8bbb9024b10
   |/   user:        test
   |    date:        Thu Jan 01 00:00:00 1970 +0000
   |    summary:     modify foo
   |
   o  changeset:   0:06e557f3edf6
      user:        test
      date:        Thu Jan 01 00:00:00 1970 +0000
      summary:     initial
   
 
   $ hg serve -p $HGPORT -d --pid-file=hg.pid -A access.log -E error.log
   $ cat hg.pid >> $DAEMON_PIDS
 
 (Try to keep these in roughly the order they are defined in webcommands.py)
 
 (log is handled by filelog/ and changelog/ - ignore it)
 
 (rawfile/ doesn't use templating - nothing to test)
 
 file/{revision}/{path} shows file revision
 
   $ request json-rev/06e557f3edf6/foo
   200 Script output follows
   
   "not yet implemented"
 
 file/{revision} shows root directory info
 
   $ request json-rev/06e557f3edf6
   200 Script output follows
   
   "not yet implemented"
 
 changelog/ shows information about several changesets
 
   $ request json-changelog
   200 Script output follows
   
   "not yet implemented"
 
 changelog/{revision} shows information about a single changeset
 
   $ request json-changelog/06e557f3edf6
   200 Script output follows
   
   "not yet implemented"
 
 shortlog/ shows information about a set of changesets
 
   $ request json-shortlog
   200 Script output follows
   
   "not yet implemented"
 
 changeset/ renders the tip changeset
 
   $ request json-rev
   200 Script output follows
   
   "not yet implemented"
 
 changeset/{revision} shows tags
 
   $ request json-rev/78896eb0e102
   200 Script output follows
   
   "not yet implemented"
 
 changeset/{revision} shows bookmarks
 
   $ request json-rev/8d7c456572ac
   200 Script output follows
   
   "not yet implemented"
 
 changeset/{revision} shows branches
 
   $ request json-rev/6ab967a8ab34
   200 Script output follows
   
   "not yet implemented"
 
 manifest/{revision}/{path} shows info about a directory at a revision
 
   $ request json-manifest/06e557f3edf6/
   200 Script output follows
   
   "not yet implemented"
 
 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
 
   $ request json-bookmarks
   200 Script output follows
   
   "not yet implemented"
 
 branches/ shows branches info
 
   $ request json-branches
   200 Script output follows
   
   "not yet implemented"
 
 summary/ shows a summary of repository state
 
   $ request json-summary
   200 Script output follows
   
   "not yet implemented"
 
 filediff/{revision}/{path} shows changes to a file in a revision
 
   $ request json-diff/f8bbb9024b10/foo
   200 Script output follows
   
   "not yet implemented"
 
 comparison/{revision}/{path} shows information about before and after for a file
 
   $ request json-comparison/f8bbb9024b10/foo
   200 Script output follows
   
   "not yet implemented"
 
 annotate/{revision}/{path} shows annotations for each line
 
   $ request json-annotate/f8bbb9024b10/foo
   200 Script output follows
   
   "not yet implemented"
 
 filelog/{revision}/{path} shows history of a single file
 
   $ request json-filelog/f8bbb9024b10/foo
   200 Script output follows
   
   "not yet implemented"
 
 (archive/ doesn't use templating, so ignore it)
 
 (static/ doesn't use templating, so ignore it)
 
 graph/ shows information that can be used to render a graph of the DAG
 
   $ request json-graph
   200 Script output follows
   
   "not yet implemented"
 
 help/ shows help topics
 
   $ request json-help
   200 Script output follows
   
   "not yet implemented"
 
 help/{topic} shows an individual help topic
 
   $ request json-help/phases
   200 Script output follows
   
   "not yet implemented"