Fixes https://github.com/mdn/content/actions/runs/5793903831/job/15702484911#step:4:22

On branch main
Your branch is up to date with 'origin/main'.

Untracked files:
(use "git add ..." to include in what will be committed)
files/en-us/web/api/rtcencodedvideoframe/

nothing added to commit but untracked files present (use "git add" to track)
Error: Process completed with exit code 1.

Problem is git -am considers only modified files. We need to use git add separately.