restic copy --batch: a fresh start from commit 382616747

Instead of rebasing my code, I decided to start fresh, since WithBlobUploader()
has been introduced.

changelog/unreleased/issue-5453:
doc/045_working_with_repos.rst:
the usual

cmd/restic/cmd_copy.go:
gather all snaps to be collected - collectAllSnapshots()
run overall copy step - func copyTreeBatched()
helper copySaveSnapshot() to save the corresponding snapshot

internal/repository/repack.go:
introduce wrapper CopyBlobs(), which passes parameter `uploader restic.BlobSaver` from
WithBlobUploader() via copyTreeBatched() to repack().

internal/backend/local/local_windows.go:
I did not touch it, but gofmt did: whitespace

internal/backend/local/local_windows.go

@@ -24,7 +24,7 @@ func removeFile(f string) error {
 	// as Windows won't let you delete a read-only file
 	err := os.Chmod(f, 0666)
 	if err != nil && !os.IsPermission(err) {
-		 return errors.WithStack(err)
+		return errors.WithStack(err)
 	}
 
 	return os.Remove(f)