include $PATH in generating list of commands for "help -a"

include $PATH in generating list of commands for "help -a"

Git had previously been using the $PATH for scripts--a previous
patch moved exec'ed commands to also use the $PATH. For consistency
"help -a" should also list commands in the $PATH.

The main commands are still listed from the git_exec_path(), but
the $PATH is walked and other git commands (probably extensions) are
listed.

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