prune: simplify code, have unified output list in reverse time order

to better visualize the "thinning out", we now list all archives in
reverse time order. at the left of each entry we tell whether we delete
or keep the archive.

previously it first listed all kept archives, then all deleted archives.

borg/archiver.py

@@ -775,13 +775,13 @@ class Archiver:
     @with_repository()
     def do_prune(self, args, repository, manifest, key):
         """Prune repository archives according to specified rules"""
-        archives = manifest.list_archive_infos(sort_by='ts', reverse=True)  # just a ArchiveInfo list
         if (args.secondly + args.minutely + args.hourly + args.daily +
             args.weekly + args.monthly + args.yearly) == 0 and args.within is None:
             self.print_error('At least one of the "keep-within", "keep-last", '
                              '"keep-secondly", "keep-minutely", "keep-hourly", "keep-daily", '
                              '"keep-weekly", "keep-monthly" or "keep-yearly" settings must be specified')
             return self.exit_code
+        archives = manifest.list_archive_infos(sort_by='ts', reverse=True)  # just a ArchiveInfo list
         if args.prefix:
             archives = [archive for archive in archives if archive.name.startswith(args.prefix)]
         keep = []
@@ -802,21 +802,21 @@ class Archiver:
         if args.yearly:
             keep += prune_split(archives, '%Y', args.yearly, keep)
 
-        keep.sort(key=attrgetter('ts'), reverse=True)
-        to_delete = [a for a in archives if a not in keep]
+        to_delete = set(archives) - set(keep)
         stats = Statistics()
         with Cache(repository, key, manifest, do_files=args.cache_files, lock_wait=self.lock_wait) as cache:
-            for archive in keep:
-                if args.output_list:
-                    logger.info('Keeping archive: %s' % format_archive(archive))
-            for archive in to_delete:
-                if args.dry_run:
-                    if args.output_list:
-                        logger.info('Would prune:     %s' % format_archive(archive))
+            for archive in archives:
+                if archive in to_delete:
+                    if args.dry_run:
+                        if args.output_list:
+                            logger.info('Would prune:     %s' % format_archive(archive))
+                    else:
+                        if args.output_list:
+                            logger.info('Pruning archive: %s' % format_archive(archive))
+                        Archive(repository, key, manifest, archive.name, cache).delete(stats)
                 else:
                     if args.output_list:
-                        logger.info('Pruning archive: %s' % format_archive(archive))
-                    Archive(repository, key, manifest, archive.name, cache).delete(stats)
+                        logger.info('Keeping archive: %s' % format_archive(archive))
             if to_delete and not args.dry_run:
                 manifest.write()
                 repository.commit(save_space=args.save_space)