Update to AMP 3.5.1
Revert back to JGit 2.0, as JGit 2.1 introduced #92

Agit v1.36 updated JGit from v2.0 to v2.2. It looks like some change was
introduced with JGit 2.1 (and all subsequent versions I've checked)
that makes the ref update fail... perhaps addressing the ref with the
wrong name (or made it sensitive to using the wrong name), as we see
debug like this:

TrackingRefUpdate : refs/heads//master old=AnyObjectId[0000000000000000000000000000000000000000] new=AnyObjectId[ce1e0703402e989bedf03d5df535401340f54b42]

The ref had a proper value before the ref update, so should not have
been 00000000... and the correct new value we see is lost, not stored
to any place I can see. Not sure what's going on with the double slash,
JGit 2.0 has it too.

Weirdly, I've only been able to reproduce this issue with repos cloned
by Agit - so 'testFetchUpdatesFromLocalTestServer()' always passes,
but 'testFetchUpdatesOnCloneFromLocalTestServer()' fails with JGit 2.1
and above.