Merge branch 'dt/commit-preserve-base-index-upon-opportunistic-cache-tree-update...

When re-priming the cache-tree opportunistically while committing
the in-core index as-is, we mistakenly invalidated the in-core
index too aggressively, causing the experimental split-index code
to unnecessarily rewrite the on-disk index file(s).

* dt/commit-preserve-base-index-upon-opportunistic-cache-tree-update:
  commit: don't rewrite shared index unnecessarily