delta-find: move pre-filtering with other pre-filtering logic
This is more consistent and will help use to be in a clean state before dealing
with the "too large group" issue.
As a side effect, the debug output now skip some useless cases, making it more useful.
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https://www.mercurial-scm.org/wiki/ContributingChanges
If you just want a checklist to follow, you can go straight to
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commits you've made (for example, `python contrib/check-commit
273ce12ad8f1` will report some style violations on a very old commit).