http: create function end_url_with_slash

http: create function end_url_with_slash

The logic to append a slash to the url if necessary in quote_ref_url
(added in 113106e "http.c: use strbuf API in quote_ref_url") has been
moved to a new function, end_url_with_slash.

The method takes a strbuf, the URL, and the path to be appended to the
URL. It first adds the URL to the strbuf. It then appends a slash
if the URL does not end with a slash.

The check on ref in quote_ref_url for a slash at the beginning has been
removed as a result of using end_url_with_slash. This check is not
needed, because slashes will be quoted anyway.

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