read-tree: shadowed variable fix.

Recent changes to built-ins broke committing from subdirectory,
because the unused parameter "prefix" shadowed a global variable.

Spotted by Jeff King.
Signed-off-by: NJunio C Hamano <junkio@cox.net>

builtin-read-tree.c

@@ -870,7 +870,7 @@ static const char read_tree_usage[] = "git-read-tree (<sha> | [[-m [--aggressive
 
 static struct lock_file lock_file;
 
-int cmd_read_tree(int argc, const char **argv, const char *prefix)
+int cmd_read_tree(int argc, const char **argv, const char *unused_prefix)
 {
 	int i, newfd, stage = 0;
 	unsigned char sha1[20];