credential: add credential.*.username

credential: add credential.*.username

Credential helpers can help users avoid having to type their
username and password over and over. However, some users may
not want a helper for their password, or they may be running
a helper which caches for a short time. In this case, it is
convenient to provide the non-secret username portion of
their credential via config.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>