Merge branch 'js/fsmonitor-unflake' into maint

The data collected by fsmonitor was not properly written back to
the on-disk index file, breaking t7519 tests occasionally, which
has been corrected.

* js/fsmonitor-unflake:
mark_fsmonitor_valid(): mark the index as changed if needed
fill_stat_cache_info(): prepare for an fsmonitor fix