remote-curl: ensure that URLs have a trailing slash

Previously, we blindly assumed that URLs passed to the remote-curl
helper did not end with a trailing slash.

Use the convenience function end_url_with_slash() from http.[ch] to
ensure that URLs have a trailing slash on invocation of the remote-curl
helper, and use the URL as one with a trailing slash throughout.

It is possible for users to pass a URL with a trailing slash to
remote-curl, by, say, setting it in remote.<name>.url in their git
config. The resulting requests have an empty path component (//) and may
break implementations of the http git protocol.

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