If we hand run_command RUN_GIT_CMD rather than 0 it will use
the execv_git_cmd path rather than execvp at the OS level.
This is typically the preferred way of running another Git
utility.
Signed-off-by: Shawn O. Pearce <spearce@spearce.org> Signed-off-by: Junio C Hamano <junkio@cox.net>