templates: add a stub template for json
Many have long wanted hgweb to emit a common machine readable output.
We start the process by defining a stub json template.
Right now, each endpoint returns a stub "not yet implemented" string.
Individual templates will be implemented in subsequent patches.
Basic tests for templates have been included. Coverage isn't perfect,
but it is better than nothing.
get-with-headers: support parsing and pretty printing JSON
Upcoming patches will add support for JSON output from hgweb.
Because JSON output from the templater is hard to read and because it
is easy to introduce malformed JSON, we introduce a JSON processing
mode to get-with-headers.py that will parse and pretty print JSON
from HTTP responses. This will make tests easier to read and write
and it will ensure hgweb is emitting well-formed JSON.