credential: add function for parsing url components

All of the components of a credential struct can be found in
a URL.  For example, the URL:

  http://foo:bar@example.com/repo.git

contains:

  protocol=http
  host=example.com
  path=repo.git
  username=foo
  password=bar

We want to be able to turn URLs into broken-down credential
structs so that we know two things:

  1. Which parts of the username/password we still need

  2. What the context of the request is (for prompting or
     as a key for storing credentials).

This code is based on http_auth_init in http.c, but needed a
few modifications in order to get all of the components that
the credential object is interested in.

Once the http code is switched over to the credential API,
then http_auth_init can just go away.

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

Documentation/technical/api-credentials.txt

@@ -67,6 +67,10 @@ Functions
 	that they may store the result to be used again.  Any errors
 	from helpers are ignored.
 
+`credential_from_url`::
+
+	Parse a URL into broken-down credential fields.
+
 Example
 -------