editor: Apply `common_prefix_len` refactor suggestion (#31957)

This adds João's nice suggestion from
https://github.com/zed-industries/zed/pull/31818#discussion_r2118582616.

Release Notes:

- N/A

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Co-authored-by: João Marcos <marcospb19@hotmail.com>
b9256dd469Gilles Peiffer committed on 6/3/2025, 9:07:14 PM· committed by GitHubparent27d3da6
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