fsmonitor: force index write after full scan

fsmonitor: force index write after full scan

fsmonitor currently only flags the index as dirty if the extension is being
added or removed. This is a performance optimization that recognizes you can
stat() a lot of files in less time than it takes to write out an updated index.

This patch makes a small enhancement and flags the index dirty if we end up
having to stat() all files and scan the entire working directory.  The assumption
being that must be expensive or you would not have turned on the feature.

Signed-off-by: Ben Peart <benpeart@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>