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Note that prune during fetch is available with JGit 3.3.0
Upgrade to JGit 3.3.1
Prepare for JGit fetch with prune

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.

Simplified branch existence assertions