Problem
Pasting a big string ("streamed paste" with multiple chunks) sets the '[
mark to the edit from the last chunk, instead of the start of the paste.
Solution:
Set the '[ mark where the paste started, not where the last chunk was
inserted.
Note: `startpos == nil` is not equal to `phase == 1` because there may
be some empty chunks pasted which won't arrive here (returned at code
before).
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