[455026] URIish: support of empty username/password

An URI like http://:@hostname:8080/tfs/Collection/TeamProject/_git/Depot is a valid
URI identifying a git depot hosted by a Microsoft TFS server.

The ':@' after the schema means "identify the current user using its system credentials".
On a MS Windows system, this will allow git to use the kerberos ticket, if any,
to authenticate against the GIT server accessible through Windows Authentication.

Signed-off-by: Ivo Bellin Salarin <me@nilleb.com>