diffutils: Merging Incomplete Lines
8.4 How 'diff3' Merges Incomplete Lines
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With '-m', incomplete lines (⇒Incomplete Lines) are simply copied
to the output as they are found; if the merged output ends in an
conflict and one of the input files ends in an incomplete line,
succeeding '|||||||', '=======' or '>>>>>>>' brackets appear somewhere
other than the start of a line because they are appended to the
incomplete line.
Without '-m', if an 'ed' script option is specified and an incomplete
line is found, 'diff3' generates a warning and acts as if a newline had
been present.