gitweb: tests faiilng due to change of default branch name

Recent versions of git change the default branch name from master to main. Gitweb tests will need to be updated.

___________________________________________ test_get_branches ____________________________________________

call_action = <function fixture_call_action.<locals>._call_action at 0x7fe934ced790>, existing_repo = None

    def test_get_branches(call_action, existing_repo):
        """Test  getting all the branches of the repository."""
>       assert json.loads(call_action(['get-branches', '--name', REPO_NAME])) == {
            "default_branch": "master",
            "branches": []
        }
E       AssertionError: assert {'branches': ...anch': 'main'} == {'branches': ...ch': 'master'}
E         Omitting 1 identical items, use -vv to show
E         Differing items:
E         {'default_branch': 'main'} != {'default_branch': 'master'}
E         Use -v to get the full diff

plinth/modules/gitweb/tests/test_actions.py:115: AssertionError