Returns the number of filler lines above line {lnum}. These are the lines that were inserted at this point in another diff'ed window. These filler lines are shown in the display but don't exist in the buffer. {lnum} is used like with getline(). Thus "." is the current line, "'m" mark m, etc. Returns 0 if the current window is not in diff mode.

Can also be used as a method:

GetLnum()->diff_filler()