connect.c: finish_connect(): allow null pid parameter

connect.c: finish_connect(): allow null pid parameter

git_connect() can return 0 if we use git protocol for example.
Users of this function don't know and don't care if a process
had been created or not, and to avoid them to check it before
calling finish_connect() this patch allows finish_connect() to
take a null pid. And in that case return 0.

[jc: updated function signature of git_connect() with a comment on
 its return value. ]

Signed-off-by: Franck Bui-Huu <vagabon.xyz@gmail.com>
Signed-off-by: Junio C Hamano <junkio@cox.net>