This PR fixes an unexpected cursor position when jumping to the
beginning of the project diff editor's first excerpt if that excerpt
starts with a deleted region. Previously, the cursor would end up in the
*following* region in this situation; now it ends up at the start of the
deleted region, as happens already for excerpts that are not the first.

Release Notes:

- N/A

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Co-authored-by: Max <max@zed.dev>