http authentication via prompts

http authentication via prompts

Curl is designed not to ask for password when only username is given in
the URL, but has a way for application to feed a (username, password) pair
to it.  With this patch, you do not have to keep your password in
plaintext in your $HOME/.netrc file when talking with a password protected
URL with http://<username>@<host>/path/to/repository.git/ syntax.

The code handles only the http-walker side, not the push side.  At least,
not yet.  But interested parties can add support for it.

Signed-off-by: Junio C Hamano <gitster@pobox.com>