Length extension attacks are not an issue for git, because every object has two fields in its header, which is prepended to the object before hashing: the object type and the length in bytes.
They've already gone through the pain, deciding on it on 2018[0] (and functional & non-experimental 3 years ago per TFA). What's left is just changing the default (and some stragglers to complete support). Changing the function now would push back changing the default by a couple additional years until the new git version gets widespread deployment (incl. on LTS distros and whatnot).
[0]: https://github.com/git/git/commit/0ed8d8da374f648764758f1303...