reftable & jgit compatibility

Thanks again for taking up this work.

As I'm browsing over your patches (and realizing how much of the arcana of the format I've forgotten), I hope that I did not make any errors in implementing the spec (and/or that Shawn didn't deviate his implementation from the spec). It would be extremely unfortunate if an incompatibility between CGit and JGit were discovered after it is released.

So far I have always been able to read JGit reftables using the C / Go
code, but it would be good to systematically test this, ie. generate a
 bunch of tables using JGit and check that passing them through the C
code (read & write) leaves them unchanged. Or perhaps check in some
tables as golden reference data.

Josh can probably connect you to the right folks to help with this on the JGit side.