Merge branch 'ss/builtin-cleanup'

"git help $cmd" unnecessarily enumerated potential command names
from the filesystem, even when $cmd is known to be a built-in.

Ideas for further optimization, primarily by killing the use of
is_in_cmdlist(), were suggested in the discussion, but they can
come as follow-ups on top of this series.

* ss/builtin-cleanup:
  builtin/help.c: speed up is_git_command() by checking for builtin commands first
  builtin/help.c: call load_command_list() only when it is needed
  git.c: consistently use the term "builtin" instead of "internal command"