http-fetch: rework url handling

http-fetch: rework url handling

Do away with a second url variable, rewritten_url, and make url
non-const. This is safe because the functions called with url (ie.
get_http_walker() and walker_fetch()) do not modify it (ie. marked with
const char *).

Also, replace code that adds a trailing slash with a call to
str_end_url_with_slash().

Signed-off-by: Tay Ray Chuan <rctay89@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>