Follow-up to https://github.com/zed-industries/zed/pull/52781, adding
some different icons to better express the state in which the split diff
_is selected_ but _isn't active_, which happens when the editor is
smaller than a given amount of defined columns.


https://github.com/user-attachments/assets/2e7aaf6c-077f-4be5-9439-ce6c2050e63d

Release Notes:

- N/A