* sp/help:
shell should call the new setup_path() to setup $PATH
include $PATH in generating list of commands for "help -a"
use only the $PATH for exec'ing git commands
list_commands(): simplify code by using chdir()
"current_exec_path" is a misleading name, use "argv_exec_path"
remove unused/unneeded "pattern" argument of list_commands
"git" returns 1; "git help" and "git help -a" return 0