extra-http-headers-v0: Feature demo: extra HTTP headers

This version of Git for Windows allows clients to send additional HTTP
headers with every HTTP request originating from Git's http:// and
https:// transport layer.

This is useful to offer temporary write access based on expiring tokens.

This example sends the additional header 'SecToken: cafebabe':

	git -c http.extraheader='SecToken: cafebabe' push origin HEAD