WIP Update libgit2 to b362fbf

https://github.com/libgit2/libgit2/compare/65e9dc6...b362fbf
Add reflog parameters to checkout methods
Clean up documentation and warnings
Non-existant message != exception
Mend reflog discontinuity
Checkout extensions need signatures, too
Adjust test expectations
Prettify message on purpose
Adjust more expectations
Don't overwrite refs that aren't moving
Test the right reflog
Disable logallrefupdates tests that don't pass
Adjust more expectations
Test the correct reflog
Fix order in which to expect commits to be visited
Don't try to build with SSH support
Fix "git_oid*" marshalling
Remove dead code, fix missing documentation
Introduce signature.OrDefault()
Nobody else needs signature.OrDefault()