Eclipse JGit: Java implementation of Git 4.6.1

Bug fixes

• GC: delete empty directories after purging loose objects
• GC.prune(Set<ObjectId>): return early if objects directory is empty
• Clean up orphan files in GC

Build and Release Engineering

• Update Jetty to 9.4.1.v20170120
• Update build to use Tycho 1.0.0
• Update Orbit to S20170120205402 and com.jcraft.jsch to 0.1.54
• Update maven plugins