[PATCH] recursive-merge: Don't print a stack trace when read-tree fails.

If the working tree is dirty read-tree will fail, and we don't want an
ugly stack trace in that case. Also make sure we don't print stack
traces when we use 'die'.

Signed-off-by: Fredrik Kuivinen <freku045@student.liu.se>
Signed-off-by: Junio C Hamano <junkio@cox.net>