Git.pm: Call external commands using execv_git_cmd()

Git.pm: Call external commands using execv_git_cmd()

Instead of explicitly using the git wrapper to call external commands,
use the execv_git_cmd() function which will directly call whatever
needs to be called. GitBin option becomes useless so drop it.

This actually means the exec_path() thing I planned to use worthless
internally, but Jakub wants it in anyway and I don't mind, so...

Signed-off-by: Petr Baudis <pasky@suse.cz>
Signed-off-by: Junio C Hamano <junkio@cox.net>