unpack-trees: skip stat on fsmonitor-valid files

unpack-trees: skip stat on fsmonitor-valid files

The index might be aware that a file hasn't modified via fsmonitor, but
unpack-trees did not pay attention to it and checked via ie_match_stat
which can be inefficient on certain filesystems. This significantly slows
down commands that run oneway_merge, like checkout and reset --hard.

This patch makes oneway_merge check whether a file is considered
unchanged through fsmonitor and skips ie_match_stat on it. unpack-trees
also now correctly copies over fsmonitor validity state from the source
index. Finally, for correctness, we force a refresh of fsmonitor state in
tweak_fsmonitor.

After this change, commands like stash (that use reset --hard
internally) go from 8s or more to ~2s on a 250k file repository on a
mac.

Helped-by: Junio C Hamano <gitster@pobox.com>
Helped-by: Kevin Willford <Kevin.Willford@microsoft.com>
Signed-off-by: Utsav Shah <utsav@dropbox.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>