Scrolling BufReadFifo to not not report final empty line

Whereas nonscrolling fifos generally[^1] append to the very end of
the buffer, scrolling fifos generally insert *before* the final
empty line.

This means that every single BufReadFifo hook in a scrolling fifo will
report an additional newline.  This is clearly wrong.  Even reporting
it only once would be wrong, because the newline is not added by a
fifo read.

This behavior has always existed for "edit -scroll -fifo" buffers.
For stdin buffers, it was re-introduced in c3b01a3c9 (Add back option
to scroll in stdin buffers, 2024-11-27).

Fix this by ending the reported range before the final empty line.

Handle one edge case: if the inserted range did not end with a
newline, the final empty line collapses into the previous line.
In this case we already don't report the newline because it's declared
"artificially-added" by 658915086 (Fix BufReadFifo overlapping range
on partial line, 2024-11-23).

This fixes the problem described at
https://github.com/mawww/kakoune/issues/5255#issuecomment-2505650511

Tests are copied verbatim from the no-scroll cases (not yet sure how
to share logic / parameterize a test in a nice way):

	$ diff -ur test/commands/fifo-read-ranges{,-scroll}
	-edit -fifo fifo *fifo*
	+edit -fifo fifo -scroll *fifo*
	$ diff -ur test/commands/fifo-read-ranges-noeol{,-scroll}
	-edit -fifo fifo *fifo*
	+edit -fifo fifo -scroll *fifo*

[^1]: unless the very last character is a fake newline (except for
      the first read, to not scroll) which we already don't report.

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