Adds 'x-tangled-repo-owner-handle' header when proxying git requests to
the knot. This allows knotserver to attempt to suggest the correct
git+ssh URL when rejecting a HTTP git push.

* Checks service name query parameter in info/refs handler; rejects any
service request that doesn't match 'git-upload-pack'.

* Produces 'text/plain' error responses so the git client will display
them.

* Adds a specific reject message for 'git-receive-pack'. This will
produce a less cryptic error in the terminal when trying to push over
http.