git over http: check for smart protocol support

https://git-scm.com/book/en/v2/Git-Internals-Transfer-Protocols

Smart Protocol support:

$ curl -s https://github.com/cweiske/anoweco.git/info/refs?service=git-upload-pack -o /dev/null -D /dev/stdout|grep -i content-type
content-type: application/x-git-upload-pack-advertisement

Dumb protocol support only:

$ curl -s https://git.cweiske.de/anoweco.git/info/refs?service=git-upload-pack -o /dev/null -D /dev/stdout|grep -i content-type
Content-Type: text/plain; charset=UTF-8