Add SmartCredentialsProvider test

An early attempt to more deeply test authentication
and credentials in the plugin.
Add CredentialsProviderImpl test, refactor existing test for some reuse
Add git version parsing test for git 2.0.0 as seen in Debian testing
Test to confirm JENKINS-23424 is a bug in our JGit implementation

JENKINS-23424 reports that a tracked file which is locally modified is
not reset to its original content on the next checkout when using our
JGit implementation.  It is reset to its original content when using
our command line git implementation.
Add test of two arg setAuthor and setCommitter
Update to ssh-credentials 1.7.1
Update to apache httpclient 4.3.4
Update to JGit 3.4.1
Merge branch 'master' of github.com:jenkinsci/git-client-plugin into remoteTagNames