diff-lib: fix check_removed when fsmonitor is on

diff-lib: fix check_removed when fsmonitor is on

`git diff-index` may return incorrect deleted entries when fsmonitor
is used in a repository with git submodules. This can be observed on
Mac machines, but it can affect all other supported platforms too.

If fsmonitor is used, `stat *st` is not initialized if cache_entry has
CE_FSMONITOR_VALID set. But, there are three call sites that rely on stat
afterwards, which can result in incorrect results.

This change partially reverts commit 4f3d6d02 (fsmonitor: skip lstat
deletion check during git diff-index, 2021-03-17).

Signed-off-by: Josip Sokcevic <sokcevic@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>