[test] force rm in the test setup
[test] Add plumbing\* tests

 * Add plumbing\blob and plumbing\tree tests
 * Adding plumbing\commit tests
 * Add plumbing\annotatedtag tests
 * Add plumbing\{head,tag} tests
[test] Let the module that requires it execute test/setup.sh
[test] Show full trace when an exception is thrown
[test] Add porcelain\tree tests
[test] More porcelain\tree tests