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
Our full contribution guidelines are in our wiki, please see:
https://www.mercurial-scm.org/wiki/ContributingChanges
If you just want a checklist to follow, you can go straight to
https://www.mercurial-scm.org/wiki/ContributingChanges#Submission_checklist
If you can't run the entire testsuite for some reason (it can be
difficult on Windows), please at least run `contrib/check-code.py` on
any files you've modified and run `python contrib/check-commit` on any
commits you've made (for example, `python contrib/check-commit
273ce12ad8f1` will report some style violations on a very old commit).