connect.c: refactor url parsing

connect.c: refactor url parsing

Make the function is_local() in transport.c public, rename it into
url_is_local_not_ssh() and use it in both transport.c and connect.c

Use a protocol "local" for URLs for the local file system.

One note about using file:// under Windows:

The (absolute) path on Unix like system typically starts with "/".
When the host is empty, it can be omitted, so that a shell scriptlet
url=file://$pwd
will give a URL like "file:///home/user/repo".

Windows does not have the same concept of a root directory located in "/".
When parsing the URL allow "file://C:/user/repo"
(even if RFC1738 indicates that "file:///C:/user/repo" should be used).

Signed-off-by: Torsten Bögershausen <tboegi@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>