debugdeltachain: protect against 0 readsize
If you don't read any data, read density should be 1.
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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
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273ce12ad8f1` will report some style violations on a very old commit).