read-tree -m -u: do not overwrite or remove untracked working tree files.

read-tree -m -u: do not overwrite or remove untracked working tree files.

When a merge results in a creation of a path that did not exist
in HEAD, and if you already have that path on the working tree,
because the index has not been told about the working tree file,
read-tree happily removes it.  The issue was brought up by Santi
Béjar on the list.

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