Problem: When iterating in reverse with {start} > {end} in
`nvim_buf_get_extmarks()`, marks that overlap {start} and are
greater than {end} are included in the return value twice.
Marks that overlap {end} and do not overlap {start} are not
not included in the return value at all. Marks are not
actually returned in a meaningful "traversal order".
Solution: Rather than actually iterating in reverse, (also possible but
requires convoluted conditions and would require fetching
overlapping marks for both the {start} and {end} position,
while still ending up with non-traversal ordered marks),
iterate normally and reverse the return value.
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