The majority of the running time of this script tends to be spent in
running git blame on source files touched by patches under review.
By introducing a git blame output cache, some of the git blame commands
don't have to re-run, and the blame information can be retrieved from a
cache.
I've observed that in a typical run matching patches available for
review with potential reviewers, this speeds up the script's running
time by a factor of about 2.5x.
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