Do not take mode bits from index after type change.

Do not take mode bits from index after type change.

When we do not trust executable bit from lstat(2), we copied
existing ce_mode bits without checking if the filesystem object
is a regular file (which is the only thing we apply the "trust
executable bit" business) nor if the blob in the index is a
regular file (otherwise, we should do the same as registering a
new regular file, which is to default non-executable).

Noticed by Johannes Sixt.

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