completion: add more parameter value completion

This adds value completion for a couple more paramters. To make it
easier to maintain these hard coded lists, add a comment at the original
list/code to remind people to update git-completion.bash too.

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

merge-recursive.c

@@ -3764,6 +3764,10 @@ int parse_merge_opt(struct merge_options *o, const char *s)
 			return -1;
 		o->merge_detect_rename = 1;
 	}
+	/*
+	 * Please update $__git_merge_strategy_options in
+	 * git-completion.bash when you add new options
+	 */
 	else
 		return -1;
 	return 0;