git blame gutter: Use smallest possible space (#18145)

Before:
![screenshot-2024-09-26-15 00
20@2x](https://github.com/user-attachments/assets/f6706325-5bef-404e-a0b4-63a5121969fa)

After:

![screenshot-2024-09-26-15 02
24@2x](https://github.com/user-attachments/assets/739d0831-0b4a-457f-917e-10f3a662e74d)


Release Notes:

- Improved the git blame gutter to take up only the space required to
display the longest git author name in the current file.

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Co-authored-by: Bennet Bo Fenner <bennet@zed.dev>