Detect truncated utf-8 characters at the end of content as still representing utf-8 (#19773)
Our character detection algorithm can potentially incorrectly detect utf-8 as iso-8859-x
if there is a truncated character at the end of the partially read file.
This PR changes the detection algorithm to truncated utf8 characters at the end of the
buffer.
Fix #19743
Signed-off-by: Andrew Thornton <art27@cantab.net>