completion: complete config variables names and values for 'git clone -c'

The previous commits taught the completion script how to complete
configuration section, variable names, and their valus after 'git -c
<TAB>', and with a bit of foresight encapsulated all that in a
dedicated helper function.  Use that function to complete the unstuck
argument of 'git config -c|--config <TAB>', which expect configuration
variables and values in the same 'section.name=value' form.

Note that handling the struck argument for 'git clone --config=<TAB>'
requires some extra care, so it will be done a separate patch.

Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/completion/git-completion.bash

@@ -1399,6 +1399,12 @@ _git_clean ()
 
 _git_clone ()
 {
+	case "$prev" in
+	-c|--config)
+		__git_complete_config_variable_name_and_value
+		return
+		;;
+	esac
 	case "$cur" in
 	--*)
 		__gitcomp_builtin clone