libgit2 based fetchGit cannot clone libsass on macOS

[pilif]

Describe the bug

libsass has had a commit happen to it that names two directories with different names which are normalized to the same name on macOS which does Unicode normalization on file names.

The PR to libsass was is https://github.com/sass/libsass/pull/2855/files and judging by the also included appveyor.xml I think the two names were not intentional.

Never the less: When libgit is cloning the repo, it behaves differently from git command line.

• git CLI doesn't check whether a directory exists when cloning and thus the working copy ends up with one directory named after whatever macOS normalizes the names to (I don't think this is good behavior, but here we are)
• libgit seems to check the file system before writing and then, after writing the first directory (after it has been normalized), will find the directory already existing when the second directory is normalized the same way.

Steps To Reproduce

builtins.fetchGit {
    url = "https://github.com/sass/libsass";
    rev = "4d229af5500be1023883c38c4a675f0ed919839d";
  }

Expected behavior

• the clone is created and the revision is checked out.

Actual behavior

error: creating directory '/nix/store/663air9nqm8jp16n6dw8f8mxwv8w3jiw-source/lib/libsass/test/e2e/unicode-pwd/Sáss-UŢF8': File exists

nix-env --version output

nix-env (Nix) 2.20.3

Additional context

In nix 2.19.3 this is still fine probably because that was before #9240 was merged.