Rewrite implements a streaming rewrite of the index that excludes the
given packs. For this it loads all index files from the repository and
only modifies those that require changes. This will reduce the index
churn when running prune. Rewrite does not require the in-memory index
and thus can drop it to significantly reduce the memory usage.
However, `prune --unsafe-recovery` cannot use this strategy and requires
a separate method to save the whole in-memory index. This is now handled
using SaveFallback.
The current in-memory index becomes stale after prune or repair index
have run. Thus, just drop the in-memory index altogether once these
commands have finished.
