Fix tab completions that start with `..`

There's an issue with tab completion expansions that causes ../ to get expanded into ./../ with each completion.

To replicate, you can do the following:

mkdir a/b/c/d/e/f/g -p
cd a

From that point, enter ../a into the terminal and press tab repeatedly. By the end, the prompt will end up with the following:

./././././././../a/b/c/d/e/f/g/