completion: add support for backwards compatibility

completion: add support for backwards compatibility

Some people might be relying on _git and _gitk to define custom aliases,
unfortunately, commit 6b179ad (completion: add new __git_complete
helper) broke that support.

  "bash: [: 1: unary operator expected"

This can be easily fixed by using __git_complete, but it's not meant to
be public.

Although _git and _gitk are probably not meant to be public, it's easy
to keep having support for them by having a wrapper to the proper
new function that is fully functional.

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