do not overwrite files marked "assume unchanged"

do not overwrite files marked "assume unchanged"

A merge will fail gracefully if it needs to update files marked
"assume unchanged", but other similar commands will not. In
particular, checkout and rebase will silently overwrite changes to
such files.

This is a regression introduced in commit 1dcafcc0 (verify_uptodate():
add ce_uptodate(ce) test), which avoids lstat's during a merge, if the
index entry is up-to-date. If the CE_VALID flag is set, however, we
cannot trust CE_UPTODATE.

Signed-off-by: Clemens Buchacher <drizzd@aon.at>
Signed-off-by: Junio C Hamano <gitster@pobox.com>