Prevent tests from using the user and system git config

This prevents tests from asking to sign dummy commits on that repo if
there's a global gpgsign config set. This also has the nice side effect
of having the same tests not pollute the user git config with random
safe directories (since run_task calls `git config --global` which tests
exercise).

Author: Eijebong

test/conftest.py

@@ -1,5 +1,7 @@
+import atexit
 import os
 import platform
+import tempfile
 from pathlib import Path
 
 import pytest
@@ -13,9 +15,14 @@
 # interference with tests.
 # This ignores ~/.hgrc
 os.environ["HGRCPATH"] = ""
-# This ignores system-level git config (eg: in /etc). There apperas to be
-# no way to ignore ~/.gitconfig other than overriding $HOME, which is overkill.
+# This ignores system-level git config (eg: in /etc).
 os.environ["GIT_CONFIG_NOSYSTEM"] = "1"
+# run-task writes to the global git config, so point it at a real
+# (writable) empty file instead of ~/.gitconfig.
+fd, git_global_path = tempfile.mkstemp(prefix="taskgraph-gitconfig-")
+os.close(fd)
+atexit.register(lambda: os.unlink(git_global_path))
+os.environ["GIT_CONFIG_GLOBAL"] = git_global_path
 
 # Some of the tests marked with this may be fixable on Windows; we should
 # look into these in more depth at some point.