The bug is easily verified by:

1. open any multi-buffer
2. place the cursor at the beginning of an excerpt
3. run the editor::ExpandExcerpts / editor: expand excerpts action
4. The excerpt is not expanded

Since the `buffer_ids_for_range` function basically did the same and had
even been changed the same way earlier I DRYed these functions as well.

Note: I'm a rust novice, so keep an extra eye on rust technicalities
when reviewing :)

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Release Notes:

- Fix editor: expand excerpts failing when cursor is at excerpt start

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Co-authored-by: Lukas Wirth <me@lukaswirth.dev>