Add a failing test case to test_git_check_version().

tests/unit/test_vcs.py

@@ -121,6 +121,7 @@ def test_git_get_src_requirements(git, dist):
 @pytest.mark.parametrize('ref,result', (
     ('5547fa909e83df8bd743d3978d6667497983a4b7', True),
     ('5547fa909', True),
+    ('5678', False),
     ('abc123', False),
     ('foo', False),
 ))