Run integration tests with all supported git versions (#2754)

Run integration tests with various different git versions, more or less
randomly picked from our range of supported versions. Based on
@Ryooooooga's work in #2459, but also restructured a bit.

All tests pass for all git versions, but only after cherry-picking
#2747.

I decided to go with @Ryooooooga's approach and do it without using
docker. I also didn't use docker locally; to reproduce the various
failures that I had to fix, I simply installed the respective git
versions locally and used something like
`PATH=~/git-versions/2.25.1/bin:$PATH
./scripts/run_integration_tests.sh`.

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