I fixed this via https://public-inbox.org/git/pull.91.git.gitgitgad...@gmail.com/T/#t and the latest word is that this will be part of v2.20.0-rc2 (which I would expect before Monday, maybe even tonight).
- Bug#914695: dgit autopkgtest breaks with git 2.20 Ian Jackson
- Bug#914695: dgit autopkgtest breaks with git 2.20 Jonathan Nieder
- Bug#914695: dgit autopkgtest breaks with git ... Ian Jackson
- Bug#914695: dgit autopkgtest breaks with ... Ian Jackson
- Bug#914695: dgit autopkgtest breaks with git 2.20 Johannes Schindelin