fsmonitor: use fsmonitor data in `git diff`

fsmonitor: use fsmonitor data in `git diff`

With fsmonitor enabled, the first call to match_stat_with_submodule
calls refresh_fsmonitor, incurring the overhead of reading the list of
updated files -- but run_diff_files does not respect the
CE_FSMONITOR_VALID flag.

Make use of the fsmonitor extension to skip lstat() calls on files
that fsmonitor judged as unmodified.

Notably, this change improves performance of the git shell prompt when
GIT_PS1_SHOWDIRTYSTATE is set.

Signed-off-by: Alex Vandiver <redacted>
Signed-off-by: Nipunn Koorapati <redacted>
Signed-off-by: Junio C Hamano <redacted>