Add the http.pinnedpubkey configuration option for public key
pinning. It allows any string supported by libcurl --
base64(sha256(pubkey)) or filename of the full public key.
If cURL does not support pinning (is too old) output a warning to the
user.
Signed-off-by: Christoph Egger <christoph@christoph-egger.org> Signed-off-by: Junio C Hamano <gitster@pobox.com>