Update the default APP_URL and add a new configuration key: GIT_URL

Signed-off-by: Pierre-Yves Chibon <pingou@pingoured.fr>

Signed-off-by: Pierre-Yves Chibon <pingou@pingoured.fr>
Use the newly created GIT_URL in the repo_info template when the repo is empty

Signed-off-by: Pierre-Yves Chibon <pingou@pingoured.fr>
Adjust the sample progit configuration file

Signed-off-by: Pierre-Yves Chibon <pingou@pingoured.fr>