Merge branch 'js/split-index-fixes'

Merge branch 'js/split-index-fixes'

The index files can become corrupt under certain conditions when
the split-index feature is in use, especially together with
fsmonitor, which have been corrected.

* js/split-index-fixes:
  unpack-trees: take care to propagate the split-index flag
  fsmonitor: avoid overriding `cache_changed` bits
  split-index; stop abusing the `base_oid` to strip the "link" extension
  split-index & fsmonitor: demonstrate a bug