Problem:
filemess() treats an empty suffix as "a buffer write is starting", but
readfile() calls it that way too. So ":read" (and ":edit", …) opens a
`nvim.bufwrite "<file>"` progress that is never completed.
Users of e.g. ghostty will see a stuck "progress" spinner.
Solution:
Only `buf_write()` starts the progress, via `filemess_progress()`.
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