Fix read-tree --prefix=dir/.

The existing code is not wrong per-se, but it started scanning the index
from a location that does not match the tree being read, and wasted
cycles.

Signed-off-by: Junio C Hamano <junkio@cox.net>

builtin-read-tree.c

@@ -228,6 +228,7 @@ int cmd_read_tree(int argc, const char **argv, const char *unused_prefix)
 		if (0 <= pos)
 			die("file '%.*s' already exists.",
 					pfxlen-1, opts.prefix);
+		opts.pos = -1 - pos;
 	}
 
 	if (opts.merge) {