Bug? clone ignores --git-dir

If you `git rebase --exec 'gulp runtests-parallel --runners user'` this project, the tests perform Git operations on various subprojects [1], but because Git exports `GIT_DIR` and `GIT_WORK_TREE`, they end up operating on the parent project instead.

Adding `--git-dir` and `--work-tree` overrides `GIT_DIR` and `GIT_WORK_TREE` [2], forcing the tests to operate on the correct repositories, however `clone` ignores `--git-dir`:

I'd expect Git to create a directory `git-dir` and put the index, etc. there, but instead it creates `directory/.git`, as usual. I'm not sure if this is a bug or the expected behavior?

[1] https://github.com/microsoft/TypeScript/blob/master/src/testRunner/externalCompileRunner.ts#L58 [2] https://github.com/microsoft/TypeScript/pull/35362