Deterministic Revisions

Every command that touches git prints real revisions - commit hashes, MERGE_HEAD, the trailing >>>>>>> <sha> on a conflict marker - and a real revision is different every time a test runs, since it’s derived from tree content, parents, and commit timestamps, none of which stay fixed between runs. Left alone, that would make every snapshot in this book flaky.

Central_test_harness fixes this by redacting: every real revision it sees is auto-detected and rewritten to a deterministic mock counterpart (see the conflict example in Import for one in the wild), so the same command run today or a year from now prints the exact same snapshot. to_mock_rev/register_rev map a real revision to its mock; redact applies that mapping - and every abbreviated prefix git might plausibly print for it - to a piece of text.

A mock revision is itself just a 40-character hex string, indistinguishable from a real one. That has one sharp edge worth pinning down directly: a mock revision can coincidentally contain a run of characters equal to some other, unrelated revision’s abbreviated prefix. redact has to substitute every registered revision starting from the original text in a single pass, so that a mock revision it has already written out is never handed back for re-examination - otherwise a later, shorter pattern could match inside it and corrupt it:

1185512b92d612b25613f2e5b473e5231185512b