shifting dependency on lib/jgit to version 3.3.0.201403021825-r.mvn.

this also includes shifting the url schema to the new central repository, and as the version name indicates, we are now pointing at a version imported from the maven artifacts (meaning filenames have slightly changed, and we're also dragging down source and docs bundles).
create test fixture package and start moving relevant code to it.

the reliance on MdmReleaseInitCmd to set up other tests is a little uncomfortable but also more or less inevitable.

i'm very unhappy about the visibility bump some random fields got in this change... it's embarassing how much of this project is still reeling from the pass-maps-everywhere python design port.  this should go away when we get to proper builder interfaces for command config.
move tmpdir logic to WithCmd to be more reusable.  TestCaseUsingRepository is now purely a wee wrapper around WithCwd.
add property for toggling whether to keep the test temp files, useful for debugging.
don't delete by default when a WithCwd is closed; it's fine for tests do inspections after leaving a cwd.  but of course, when a whole test method is done, clearing is a reasonable default.
create a fixture for a simple project repo.
create a helper method for listing files in a repo at a ref; seems like it's going to come up a lot in crafting tests.
add a sanity check to the projectAlpha fixture setup.

note that nested classes declaring tests are not run by the ant 'run-test' target.  since this is more for a sanity check against outright wrong programming than a useful regression test, that's acceptable.