fetchTarball fails after downloading proton-ge

[babbaj]

Describe the bug

In my config I've been using fetchTarball to download proton-ge builds but after updating to nix 2.21.0 I am now getting this error from fetchTarball
error: adding a file to a tree builder: failed to insert entry: invalid name for a tree entry - .git

Steps To Reproduce

run nix repl --expr 'fetchTarball { url = "https://github.com/GloriousEggroll/proton-ge-custom/releases/download/GE-Proton9-4/GE-Proton9-4.tar.gz"; sha256 = "sha256-AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA="; }'
(older versions have the same problem)

Expected behavior

fetchTarball should be able to unpack the tar file with no errors

nix-env --version output

babbaj@nixos:~ ❯ nix --version
nix (Nix) 2.21.0

Additional context

There is a .git folder included in the protonfixes subdirectory which is probably what is triggering the error.
I believe the error comes specifically from here

nix/src/libfetchers/git-utils.cc

Line 820 in 8c4c215

throw Error("adding a file to a tree builder: %s", git_error_last()->message);

It doesn't make sense to me why git is involved in fetchTarball when it should and seemingly has been able to unpack any abitrary files in stable releases.

This commit is most likely what introduced this regression.
cabee98

proton-ge was also added to nixpkgs recently and uses fetchzip instead of fetchTarball. Using fetchzip in place of fetchTarball works fine.

Priorities

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[thufschmitt]

cc @edolstra . I can confirm this, and cabee98 is a very likely culprit indeed. If we want to keep this git hashing thing, we'll need to be able to bail out of work around this kind of file names that aren't accepted by libgit2