jsonchangeset: set manifest node to "null" for workingctx

Unlike changeset_printer, it does not hide the manifest field because JSON
output will be parsed by machine where explicit "null" will be more useful
than nothing.

mercurial/cmdutil.py

@@ -1296,7 +1296,11 b' class jsonchangeset(changeset_printer):'
                       ", ".join('"%s"' % c.hex() for c in ctx.parents()))
 
         if self.ui.debugflag:
-            self.ui.write(',\n  "manifest": "%s"' % hex(ctx.manifestnode()))
+            if rev is None:
+                jmanifestnode = 'null'
+            else:
+                jmanifestnode = '"%s"' % hex(ctx.manifestnode())
+            self.ui.write(',\n  "manifest": %s' % jmanifestnode)
 
             self.ui.write(',\n  "extra": {%s}' %
                           ", ".join('"%s": "%s"' % (j(k), j(v))

tests/test-log.t

@@ -1666,6 +1666,27 b' working-directory revision requires spec'
    }
   ]
 
+  $ hg log -r 'wdir()' -Tjson --debug
+  [
+   {
+    "rev": null,
+    "node": null,
+    "branch": "default",
+    "phase": "draft",
+    "user": "test",
+    "date": [*, 0], (glob)
+    "desc": "",
+    "bookmarks": [],
+    "tags": ["tip"],
+    "parents": ["65624cd9070a035fa7191a54f2b8af39f16b0c08"],
+    "manifest": null,
+    "extra": {"branch": "default"},
+    "modified": [],
+    "added": [],
+    "removed": []
+   }
+  ]
+
 Check that adding an arbitrary name shows up in log automatically
 
   $ cat > ../names.py <<EOF