credentials: add "store" helper

credentials: add "store" helper

This is like "cache", except that we actually put the
credentials on disk. This can be terribly insecure, of
course, but we do what we can to protect them by filesystem
permissions, and we warn the user in the documentation.

This is not unlike using .netrc to store entries, but it's a
little more user-friendly. Instead of putting credentials in
place ahead of time, we transparently store them after
prompting the user for them once.

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