In multi-buffer context, `do_completion` resolved the completion's
replace range anchors against whichever buffer currently held the newest
selection. If that selection had moved into a different file's excerpt
(while the menu remained open), the anchors belonged to a different
buffer and caused a panic.

This PR adds a guard to confirm the completion buffer and selection
buffer is the same.

Closes FR-76.

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Release Notes:

- Fixed a panic that could occur when confirming a completion in a
multibuffer if the cursor had moved into a different file