The size comparison for `--max-unused` only accounted for unused but not
for duplicate data. For repositories with a large amount of duplicates
this can result in a situation where no data gets pruned even though
the amount of unused data is much higher than specified.
Those methods now only allow modifying snapshots. Internal data types
used by the repository are now read-only. The repository-internal code
can bypass the restrictions by wrapping the repository in an
`internalRepository` type.
The restriction itself is implemented by using a new datatype
WriteableFileType in the SaveUnpacked and RemoveUnpacked methods. This
statically ensures that code cannot bypass the access restrictions.
The test changes are somewhat noisy as some of them modify repository
internals and therefore require some way to bypass the access
restrictions. This works by capturing an `internalRepository` or
`Backend` when creating the Repository using a test helper function.
Pack files created by interrupted prune runs, appear to consist only of
duplicate blobs on the next run. This caused the previous heuristic to
ignore those pack files. Now, a duplicate blob in a specific pack file
is also selected if that pack file only contains duplicate blobs. This
allows prune to select the already rewritten pack files.
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.
