tests: try harder to prevent Git from reading external configuration

Tests would fail if ~/.gitconfig contained for example:

[init]
defaultBranch = main

Setting GIT_CONFIG_GLOBAL=/dev/null prevents Git from reading ~/.gitconfig ...
but isn't support by Dulwich and doesn't solve the problem.

We will thus work around this issue by setting HOME to the temp test location.
That will prevent Dulwich from reading configuration files by making
os.path.expanduser point to a location with known content.

We also set XDG_CONFIG_HOME to avoid reading ~/.config/git/config .

Setting GIT_CONFIG_NOSYSTEM will prevent reading /etc/gitconfig - that seems
like a good idea too.