gather git history for translations

[fiji-flo]

No description provided.

[peterbe]

Now that there are so many files written, the loudness of printing each one and the green makes it quite "useless".

Would you mind doing something similar to what we did for node tool/cli.js spas
So perhaps it can simply just print out:

Wrote 29 _githistory.json files

[peterbe]

Not sure if it's a nit or not but would you mind tuning the CLI output as mentioned in the comment above?

I knew this landed about 8h ago. So I would expect it to say "2021/03/10" in some French i18n formatting.
But instead I get:

But then it hit me! Julien merged it 8h ago, but the author authored it about 10 days ago.
So it really makes me wonder if we're doing the right thing.
From a reader's point of view, they see "Feb 28" on there but that's not relevant or interesting because the content is different today compared to yesterday.
I think we should look into use the merge date instead of the commit date. Separate issue for sure but it's relevant. I'll file something.

I noticed that the _githistory.json file, for fr, is full of entries for every single old name. E.g.

  "fr/accessibilité/aperçu_d_applications_web_et_de_composants_dynamiques_accessibles/index.html": {
    "modified": "2020-12-08T19:40:17.000Z",
    "hash": "33058f2b292b3a581333bdfb21b8f671898c5060"
  },
  "fr/accessibilité/aria/comment_deposer_un_bug_lie_a_aria/index.html": {
    "modified": "2020-12-08T19:40:17.000Z",
    "hash": "33058f2b292b3a581333bdfb21b8f671898c5060"
  },

These names will never be useful. I don't know if it's worth optimizing for but we could kill/bail the git log loop for anything older than 2021/02/11 because anything before the first en-slug sync is never going to match.

Also, don't forget to add _githistory.json to the .gitigore in mdn/translated-content.

[peterbe]

I filed #3189 as a followup.

[peterbe]

With nit.