Merge branch 'dg/subtree-test-cleanup'

Test cleanups for the subtree project. * dg/subtree-test-cleanup: contrib/subtree: Handle '--prefix' argument with a slash appended contrib/subtree: Make each test self-contained contrib/subtree: Add split tests contrib/subtree: Add merge tests contrib/subtree: Add tests for subtree add contrib/subtree: Add test for missing subtree contrib/subtree: Clean and refactor test code