re-introduce GitAPI(String,File,TaskListener,EnvVars,String)
for backward compatibility
split IGitAPI

  * GitClient for supported methods, to be later implemented using JGit
  * IGitAPI for backward compatibility

Provide a builder-style factory to create instances, so that the plugin can later plug a customizable strategy to resolve the implementation without impact on plugin that uses git-client.
fix broken test
restore GitAPI#showRevision(Revision)
and GitAPI#revList(reference)
used by git-parameter-plugin
make JGit implementation a pure Java implementation
disable tests on JGit client for unimplemented methods