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 method is not available on the restic.Repository interface that is
used for testing. Drop the call as a small amount of additional index
writes is not a problem.
Currently, the cmd/restic package contains a significant amount of code
that modifies repository internals. This code should in the mid-term
move into the repository package.
