Master use JGit http from git client plugin

As noted in [JENKINS-21756], it is more complicated than necessary to have a direct dependency on JGit http server in git-server.  Instead, as Kohsuke suggested, it is simpler if git-client-plugin includes the JGit http component so that git-server (and others if needed) can depend on git-client-plugin without requiring that they include (and track) their own version of JGit.