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>