http: use hostname in credential description

http: use hostname in credential description

Until now, a request for an http password looked like:

  Username:
  Password:

Now it will look like:

  Username for 'example.com':
  Password for 'example.com':

Picked-from: Jeff King <peff@peff.net>
Signed-off-by: Michael J Gruber <git@drmicha.warpmail.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>