Accept IOException from temporaryDirectoryAllocator, don't fail test

The previous code checked for a Jenkins specific exception, but JDK 8
(or newer Jenkins versions needed for JDK 8) throw a standard Java
IOException rather than the Jenkins specific IOException2.
[Fix JENKINS-22343] JGit now shows first revision in repo correctly

The first revision in the repository has no parent.  The code assumed
all commits have at least one parent.
Add GitTool test
Update to JGit 3.4.0
[maven-release-plugin] prepare release git-client-1.9.0
[maven-release-plugin] prepare for next development iteration
Remove unused revTree variable from JGitAPIImpl