corrected issue with multiple line undo at a new line

if the change occurred at a new line, the code was incorrectly assuming
that no lines were deleted (which occurs for backspace or delete),
however, the change could also have been due to a multiple line undo;
therefore, instead of resetting the net line count to zero, the net line
count is incremented (one less line deleted) and only if 0 should no
lines be reported as deleted