convert all throws clauses in FixtureUtil methods for setup into fatal errors.  I'm not interesting in propagating signatures of unreasonable failures of setup code throughout tests.
actually, make a unique error type to indicate that a failure is something drastic in fixture setup (as opposed to something other parts of the system failed to handle)
apply the SystemReaderFilteringProxy hack/fix in our test case superclass (that "Invalid value: diff.algorithm=patience" thing where the contents of my homedir break jgit (sigh) is getting on my nerves again).
automatically prepare dirs as necessary when using WithCwd.
grammar fix in error message.
using the standard test superclass in ProjectAlpha.SanityCheck doesn't have to do its own tmpdiring.
create fixture for a simple release repo.

as with the introduction project repo fixture, some random fields in a command got a visibility bump, and this is ishy but should go away when we get to proper builder interfaces for command config.
remove tons of setup logic from TestMdmAddCommand and use the new fixtures system.  most of the SLOC in this test is now actually about the test; much improved.
typo fix
prefer throwing to returning MdmExitMessage when it's an nonsuccess
add test for MdmAddCommand failing on a version that doesn't exist
add test for MdmAddCommand against a release repo with multiple versions.  introduces fixture ProjectBetaReleases.
tis a silly place to put a resource cleanup.  not that it matters; the parent is deleted at the end of the test anyway.
add test for MdmAddCommand against a nonsense release repo url.
create test for MdmUpdateCommand operating on a repo after clone.