Git path completion has extra ambiguity

```fish
git init foo
cd foo/
mkdir bar/
touch bar/a
git add .
```

Then compare the behaviour of `ls ` and `git log -- `. The former completes `bar/` unambiguously, but the latter prompts you to choose from `bar/` and `bar/a`. If you are used to path completions in fish outside of Git, then this behaviour is a bit surprising.

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fish 4.7.1