As mentioned in another comment, protocol v2 was implemented by a Google employee, and they decided to write a blog post about it. This is not an official git announcement.
> It just works and during all these years my system was never left broken after an update
I find that very hard to believe. While I am still running Arch (gotta love those shiny new features), updates did break something minor fairly frequently due to occasionally incompatible updates, e.g. a missing symbol in a shared object.
Luckily there was only one time where something major broke which took me a few hours to fix.
If you can upgrade you can use "git stash -p" with pathspecs as well, so you can stash some changes in a specific file without having to go through changes in other files.
You don't really have to roll your own crypto to create such an app. There's always openssl and the signal protocol, which you'd only need to implement without designing anything.
Sure that can go wrong as anything can, but it's far from rolling your own crypto and makes things a lot easier.
I feel like this still ignores most of what Linus said on why git isn't broken. In particular "it's fairly trivial to detect the fingerprints of using this attack" in his Google+ post. https://plus.google.com/+LinusTorvalds/posts/7tp2gYWQugL
And there are already patches on the mailing list for that.