revlog: add a way to keep track of older uids in the docket
When the revlog content is rewritten, we will use new files, to avoid truncating
the previous ones. We need some way to keep track of the older files before we
clean them up.
Differential Revision: https://phab.mercurial-scm.org/D10866
Hello, Foobar
[b'Hello, Fo, =;:<o', b'Hello, Bar']