http: do not ignore proxy path

http: do not ignore proxy path

The documentation for `http.proxy` describes that option, and the
environment variables it overrides, as supporting "the syntax understood
by curl". curl allows SOCKS proxies to use a path to a Unix domain
socket, like `socks5h://localhost/path/to/socket.sock`. Git should
therefore include, if present, the path part of the proxy URL in what it
passes to libcurl.

Co-authored-by: Jeff King <peff@peff.net>
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Ryan Hendrickson <ryan.hendrickson@alum.mit.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>