use only the $PATH for exec'ing git commands

use only the $PATH for exec'ing git commands

We need to correctly set up $PATH for non-c based git commands.
Since we already do this, we can just use that $PATH and execvp,
instead of looping over the paths with execve.

This patch adds a setup_path() function to exec_cmd.c, which sets
the $PATH order correctly for our search order. execv_git_cmd() is
stripped down to setting up argv and calling execvp(). git.c's
main() only only needs to call setup_path().

Signed-off-by: Scott R Parish <srp@srparish.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>