Return parsed objects from TestRepository.commit/tree/blob()

Return parsed objects from TestRepository.commit/tree/blob()

It is convenient for TestRepository to return fully parsed
objects from its commit()/tree()/blob() methods, so that test
code doesn't have to remember to parse them before making
assertions about them.

Update TestRepostiory to return fully parsed objects.
Adjust the tests that are affected by this change in behavior.

Change-Id: I09d03d0c80ad22cb7092f4a2eaed99d40a10af63
Signed-off-by: Terry Parker <tparker@google.com>