signed-push: add "pushee" header to push certificate

signed-push: add "pushee" header to push certificate

Record the URL of the intended recipient for a push (after
anonymizing it if it has authentication material) on a new "pushee
URL" header.  Because the networking configuration (SSH-tunnels,
proxies, etc.) on the pushing user's side varies, the receiving
repository cannot know the single canonical URL all the pushing
users would refer it as (besides, many sites allow pushing over
ssh://host/path and https://host/path protocols to the same
repository but with different local part of the path), the value
cannot be reliably used for replay-attack prevention purposes (which
is solved separately by the "nonce" mechanism), but this will serve
as a human readable hint to identify the repository the certificate
refers to.

Signed-off-by: Junio C Hamano <gitster@pobox.com>