Adapt to the JGit v4.0.0.201506090130-r

This version of JGit also removes the 'release()' method in many
interfaces/classes in favor of implementing the AutoCloseable interface.
In stable-2.10 we just replace all usages of the release() method with
the close() method.

Change-Id: I6af2149b337d3a817cb710ae8901d9582098730a