Skara should ignore .gitconfig and /etc/gitconfig

Many git users are configuring init.defaultBranch to some value other than "master", usually "main", perhaps by adding to /etc/gitconfig

```
[init]
defaultBranch = main
```

This causes many tests to fail, e.g.

```
RepositoryTests > [1] GIT FAILED
    org.opentest4j.AssertionFailedError: expected: <[master]> but was: <[main]>
```

When I reset this with e.g.
git config --global init.defaultBranch master; sh gradlew :vcs:test
the results for :vcs:test go from

281 tests completed, 33 failed, 11 skipped
to
281 tests completed, 11 failed, 11 skipped

Likewise, you can surely repro using

```
git config --global init.defaultBranch main
```