Fix testing/test_git.py::test_git_archive_export_ignore

Ensure `$HOME` is unset, since otherwise `init.templatedir` might get
picked up from the user's config, which might e.g. not include the
`.git/info` dir then.

Author: blueyed

testing/test_git.py

@@ -10,7 +10,8 @@
 
 
 @pytest.fixture
-def wd(wd):
+def wd(wd, monkeypatch):
+    monkeypatch.delenv("HOME", raising=False)
     wd("git init")
     wd("git config user.email [email protected]")
     wd('git config user.name "a test"')