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

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