Hi Mene,
I'd go with the cron-job, to trigger git add and git commit. ... eg: You could create a "special tiddler" named commit.txt, with a commit message.
The cron job can scan for this tiddler.
If the cron job finds this tiddler it takes the text and removes the commit.txt and commit.txt.meta from the directory and
triggers the git actions.
This way you can easily create git commits only if you are sure, that your stuff is valid. ... If the sync adapter will trigger every change you make, you'll have a lot of "temporary" content being part of your git repo.
just my thoughts
mario