To really omit the added lines, you will have to write a custom diff function, cp. :help diff-diffexpr.
If you just don’t want to see the added lines (because they are too visually distracting), you can change the selection group DiffAddto display white on white (or black on black in the console) text (or any other low-contrast coloring in the same way:
:highlight DiffAdd ctermfg=black ctermbg=NONE guifg=bg guibg=NONE
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