completion: put main git and gitk completion functions back into git namespace

completion: put main git and gitk completion functions back into git namespace

Commit 7f02f3d7 (completion: rename internal helpers _git and _gitk,
2012-05-19) renamed said functions to _main_git() and _main_gitk(),
respectively.  By convention the name of our git-completion-specific
functions start with '_git' or '__git' prefix, so rename those
functions once again to put them back into our "namespace".  Use the
two underscore prefix, because _git_main() could be mistaken for the
completion function of the (not yet existing) 'git main' command.

Signed-off-by: SZEDER Gábor <szeder@ira.uka.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>