hgweb: add "children" into the JSON template for a changeset

The children are shown in the HTML templates, so I don’t see a reason against including them in the JSON template as well.

mercurial/templates/json/map

@@ -65,6 +65,7 b" changeset = '\\{"
   "tags": [{join(changesettag, ", ")}],
   "user": {author|utf8|json},
   "parents": [{join(parent%changesetparent, ", ")}],
+  "children": [{join(child%changesetparent, ", ")}],
   "files": [{join(files, ", ")}],
   "diff": [{join(diff, ", ")}],
   "phase": {phase|json}

tests/test-hgweb-json.t

@@ -777,6 +777,7 b' changeset/ renders the tip changeset'
   {
     "bookmarks": [],
     "branch": "default",
+    "children": [],
     "date": [
       0.0,
       0
@@ -809,6 +810,9 b' changeset/{revision} shows tags'
   {
     "bookmarks": [],
     "branch": "default",
+    "children": [
+      "93a8ce14f89156426b7fa981af8042da53f03aa0"
+    ],
     "date": [
       0.0,
       0
@@ -897,6 +901,9 b' changeset/{revision} shows bookmarks'
       "bookmark1"
     ],
     "branch": "default",
+    "children": [
+      "78896eb0e102174ce9278438a95e12543e4367a7"
+    ],
     "date": [
       0.0,
       0
@@ -957,6 +964,9 b' changeset/{revision} shows branches'
   {
     "bookmarks": [],
     "branch": "test-branch",
+    "children": [
+      "ed66c30e87eb65337c05a4229efaa5f1d5285a90"
+    ],
     "date": [
       0.0,
       0