The codepath to support the experimental split-index mode had
remaining "racily clean" issues fixed.
* sg/split-index-racefix:
split-index: BUG() when cache entry refers to non-existing shared entry
split-index: smudge and add racily clean cache entries to split index
split-index: don't compare cached data of entries already marked for split index
split-index: count the number of deleted entries
t1700-split-index: date back files to avoid racy situations
split-index: add tests to demonstrate the racy split index problem
t1700-split-index: document why FSMONITOR is disabled in this test script