completion: don't offer commands when 'git --opt' needs an argument

The main git options '--git-dir', '-c', '-C', '--worktree' and '--namespace' require an argument, but attempting completion right after them lists git commands. Don't offer anything right after these options, thus let Bash fall back to filename completion, because - the three options '--git-dir', '-C' and '--worktree' do actually require a path argument, and - we don't complete the required argument of '-c' and '--namespace', and in that case the "standard" behavior of our completion script is to not offer anything, but fall back to filename completion. Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>