Include jgit http server jar in git-client-plugin for API consumers

[JENKINS-21756] shows that JGit version mismatches can complicate the
management of plugins. Including JGit http server in this plugin is
hoped to reduce the version mismatch cases.
Update to 1.11.0-SNAPSHOT, larger changes justify the switch

Updating to JGit 3.5.1, dependencies on newer versions of other
plugins, etc.
Merge commit 'c82fbe31e9906c5577555140273cd17f0e699cfd' into HEAD