Switch from unreleased JGit 3.4.0 to released JGit 3.3.2

My sincere apologies that I did not detect the "-m7" at the end of the
JGit version number string.  That means we've delivered an unreleased
JGit version to Jenkins users since git-client-plugin 1.9.0.  I don't
think that causes catastrophic harm, since I've seen no JGit related
bug reports since the git client plugin 1.9.0 release.