Prompt for a username when an HTTP request 401s

Prompt for a username when an HTTP request 401s

When an HTTP request returns a 401, Git will currently fail with a
confusing message saying that it got a 401, which is not very
descriptive.

Currently if a user wants to use Git over HTTP, they have to use one
URL with the username in the URL (e.g. "http://user@host.com/repo.git")
for write access and another without the username for unauthenticated
read access (unless they want to be prompted for the password each
time). However, since the HTTP servers will return a 401 if an action
requires authentication, we can prompt for username and password if we
see this, allowing us to use a single URL for both purposes.

This patch changes http_request to prompt for the username and password,
then return HTTP_REAUTH so http_get_strbuf can try again.  If it gets
a 401 even when a user/pass is supplied, http_request will now return
HTTP_NOAUTH which remote_curl can then use to display a more
intelligent error message that is less confusing.

Signed-off-by: Scott Chacon <schacon@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>