GitTool configuration moved from git-plugin
[JENKINS-17013] preventive workaround 
warn and return default installation if requested name don't match any registered installation
don't set a default exe so that JGit is use if not explicitly set
implement isCommitInRepo with JGit
implement revParse with JGit
implement hasGitRepo with JGit
implement deleteTag with JGit
implement getTagMessage with JGit
attempt to implement clean with JGit (WIP)