The tricky thing about this turned out to be handling renames and reverts.
For that, it has to make two passes over the git log, and to avoid
buffering a possibly huge amount of logs in memory (ie the whole git log of
an entire repository!), runs git log twice.
(It might be possible to speed this up by asking git log to show a diff,
and so avoid needing to use catKey.)
Sponsored-By: Brock Spratlen on Patreon
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