snapshot: make sure we'll never refine delta base from a reused source
The point of reusing delta from the source is to avoid doing computation when
applying a bundle. Refining such delta would go against that spirit.
We do not have refining logic in place yet. This code needed to be moved out
of the way before we could start adding such logic.
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).