[find_interesting_reviews.py] Add git blame output cache

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.