Improve memory use of --all when using annex.private

Improve memory use of --all when using annex.private

This does not improve Annex.Branch.files at all, since it still uses ++ to
combine the lists, so forcing all but the last one.

But when there are a lot of files in the private journal, it does avoid
--all (or a bare repo) from buffering the filenames in memory.

See commit 653b7194727a27dfd185cdb0b752f8e190250fc4 for prior discussion of
this buffering.

Sponsored-by: Graham Spencer on Patreon