generate-cmdlist.sh: collect config from all config.txt files

This script uses Documentation/config.txt as input for "git help
--config" and "git config" completion but it misses the fact that
config.txt includes other txt files. Include all *config.txt as input
when scanning for config keys. This could produce false positives, but
as long as we stick to the blah-config.txt naming convention, we
should be ok.

While at there, move diff.* from config.txt to diff-config.txt where
all other diff config keys are.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

generate-cmdlist.sh

@@ -80,7 +80,7 @@ print_config_list () {
 	cat <<EOF
 static const char *config_name_list[] = {
 EOF
-	grep '^[a-zA-Z].*\..*::$' Documentation/config.txt |
+	grep -h '^[a-zA-Z].*\..*::$' Documentation/*config.txt |
 	sed '/deprecated/d; s/::$//; s/,  */\n/g' |
 	sort |
 	while read line