Simplify branch existence assertions, disable fetch with prune test for JGit

The fetch with prune operation in JGit 3.3.0 and 3.3.1 deletes more
branches than command line git.  Don't enable fetch with prune for
JGit until further investigation has decided if that difference in
behavior is acceptable.
Disable JGit prune during fetch - prunes too many branches in JGit 3.3.0 and 3.3.1