Returns the highlight ID for diff mode at line {lnum} column {col} (byte index). When the current line does not have a diff change zero is returned. {lnum} is used like with |getline()|. Thus "." is the current line, "'m" mark m, etc. {col} is 1 for the leftmost column, {lnum} is 1 for the first line. The highlight ID can be used with |synIDattr()| to obtain syntax information about the highlighting. Can also be used as a |method|: GetLnum()->diff_hlID(col)