connect: clear child process before freeing in diagnostic mode

connect: clear child process before freeing in diagnostic mode

The git_connect() function has a special CONNECT_DIAG_URL mode, where we
stop short of actually connecting to the other side and just print some
parsing details. For URLs that require a child process (like ssh), we
free() the child_process struct but forget to clear it, leaking the
strings we stuffed into its "env" list.

This leak is triggered many times in t5500, which uses "fetch-pack
--diag-url", but we're not yet ready to mark it as leak-free.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>