completion: add proper public __git_complete

completion: add proper public __git_complete

When __git_complete was introduced, it was meant to be temporarily, while
a proper guideline for public shell functions was established
(tentatively _GIT_complete), but since that never happened, people
in the wild started to use __git_complete, even though it was marked as
not public.

Eight years is more than enough wait, let's mark this function as
public, and make it a bit more user-friendly.

So that instead of doing:

  __git_complete gk __gitk_main

The user can do:

  __git_complete gk gitk

And instead of:

  __git_complete gf _git_fetch

Do:

  __git_complete gf git_fetch

Backwards compatibility is maintained.

Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>