bug#7041: comint seems to truncate from places other than the top

[Reuben Thomas]

If I add comint-truncate-buffer to comint-output-filter-functions, and
set comint-buffer-maximum-size to 1024, then when a term buffer
reaches about that size, and I run a command that produces a lot of
output, then scroll back, some of the output is missing. In other
words, rather than truncate from the top of the buffer, some of the
truncation at least seems to happen much nearer the bottom.

If I remove comint-truncate-buffer from
comint-output-filter-functions, or run a command that produces a lot
of output in a fresh window (so that the number of lines of output
does not exceed comint-buffer-maximum-size) then I do not see this
behaviour, which is why I suspect it is to do with truncation.

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