https://bugs.freedesktop.org/show_bug.cgi?id=111077
--- Comment #39 from Matt Turner mattst88@gmail.com --- (In reply to roland@rptd.ch from comment #38)
Could it be that your patch is not for version "mesa-19.0.8" as I'm using? (newer ones are keyworded)
Perhaps try 19.2.0_rc3. That's much closer to master.
Whatever the case, if the patch doesn't apply to 19.0.8 and portage tried (and as a result failed) to apply it, it would give an error and stop.