Merge branch 'sp/help'

Merge branch 'sp/help'

* 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