Thanks for sharing that information. The git plugin includes an optional "pure java" implementation which can be enabled from the "Manage Jenkins" and "Configure System" page. The user select "Git" and then adds "JGit" as the implementation from the pick list.
The JGit implementation does not handle all the use cases which command line git handles, but it handles the most critical use cases.
Most of the cases which are not handled by JGit are described on the Git plugin wiki page on jenkinsci.org