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