Firefox has a new git repo with a completely different history

Hi!

Mozilla decided to switch from Mercurial to Git, which is great, except for one tiny detail: rather than building upon the existing Git mirror (gecko-dev), they created a brand new repository whose history is completely unrelated.

When I added the new remote to my local clone, its .git grew to 12GB... My local clone is probably bigger than some other clones, but I expect that if we do it with the current tor-browser.git, its .git size will grow at least at 10GB.

We're tracking this in tpo/applications/tor-browser#43660.