Fix "git clone" for git:// protocol

Fix "git clone" for git:// protocol

In ba227857(Reduce the number of connects when fetching), we checked
the return value of git_connect() to see if the connection was
successful.

However, for the git:// protocol, there is no need to have another
process, so the return value was NULL.

Now, it makes sense to assume the rule that git_connect() will return
NULL if it fails (at the moment, it die()s if it fails), so return
a dummy child process.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Acked-by: Daniel Barkalow <barkalow@iabervon.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>