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>

contrib/completion/git-completion.bash

@@ -2808,6 +2808,17 @@ __git_main ()
 	done
 
 	if [ -z "$command" ]; then
+		case "$prev" in
+		--git-dir|-C|--work-tree)
+			# these need a path argument, let's fall back to
+			# Bash filename completion
+			return
+			;;
+		-c|--namespace)
+			# we don't support completing these options' arguments
+			return
+			;;
+		esac
 		case "$cur" in
 		--*)   __gitcomp "
 			--paginate