Do not unquote + into ' ' in URLs

Do not unquote + into ' ' in URLs

Since 9d2e942 (decode file:// and ssh:// URLs, 2010-05-23) the URL
logic unquotes escaped URLs.  For the %2B type of escape, this is
conformant with RFC 2396.  However, it also unquotes + into a space
character, which is only appropriate for the query strings in HTTP.
This notably broke fetching from the gtk+ repository.

We cannot just remove the corresponding code since the same
url_decode_internal() is also used by the HTTP backend to decode query
parameters.  Introduce a new argument that controls whether the +
decoding happens, and use it only in the (client-side) url_decode().

Reported-by: Jasper St. Pierre <jstpierre@mecheye.net>
Signed-off-by: Thomas Rast <trast@student.ethz.ch>
Signed-off-by: Junio C Hamano <gitster@pobox.com>