remote-curl: send Accept-Language header to server

remote-curl: send Accept-Language header to server

Git server end's ability to accept Accept-Language header was introduced
in f18604bbf2 (http: add Accept-Language header if possible, 2015-01-28),
but this is only used by very early phase of the transfer, which is HTTP
GET request to discover references. For other phases, like POST request
in the smart HTTP, the server does not know what language the client
speaks.

Teach git client to learn end-user's preferred language and throw
accept-language header to the server side. Once the server gets this header,
it has the ability to talk to end-user with language they understand.
This would be very helpful for many non-English speakers.

Helped-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Li Linchao <lilinchao@oschina.cn>
Signed-off-by: Junio C Hamano <gitster@pobox.com>