externals: Checkout 79e8d17024 for FFmpeg
6b6b9e593d does not exist on FFmpeg master, and tag n4.3.1 requires
manually fetching all of FFmpeg's tags. `git` reports that the commit
does not exist initially and can be confusing as a result. Instead,
checkout the immediately previous commit from n4.3.1 on their master
branch.
lat9nq