I understand that the git plugin is now using git-client?

I can't get my upgraded Jenkins instance to clone a repository at all.
I've tried adding
-Dorg.jenkinsci.plugins.gitclient.Git.useCLI=true
-Dhudson.util.ProcessTree.disable=true

I've also tried setting environment variables (GIT_HOME and HOME) in Jenkins as per the details on this page.
https://wiki.jenkins-ci.org/display/JENKINS/Git+Plugin

I've set the tool location of Git (gitk.cmd) .
GitInstallations no longer works (I assume because of the use of Git-Client now)

Log output from job looks like this:
Checkout:workspace / C:\Jenkins\jobs\Maintenance_CollectArtefactVersions\workspace - hudson.remoting.LocalChannel@2b498ab1
Using strategy: Default
Cloning the remote Git repository
Cloning repository ssh://git@server.com:7999/Repos/deployscripts.git
git --version
Process leaked file descriptors. See http://wiki.jenkins-ci.org/display/JENKINS/Spawning+processes+from+build for more information
Fetching upstream changes from origin
FATAL: Invalid id: Process leaked file descriptors. See http://wiki.jenkins-ci.org/display/JENKINS/Spawning+processes+from+build for more information
java.lang.IllegalArgumentException: Invalid id: Process leaked file descriptors. See http://wiki.jenkins-ci.org/display/JENKINS/Spawning+processes+from+build for more information
at org.eclipse.jgit.lib.ObjectId.fromString(ObjectId.java:232)
at org.jenkinsci.plugins.gitclient.CliGitAPIImpl.revParse(CliGitAPIImpl.java:264)
at hudson.plugins.git.GitAPI.revParse(GitAPI.java:252)

I'm still kind of assuming I'm doing something wrong now that git is using git-client, or need to remove something given how much use of Jenkins+Git I've had in the past (3+years on ~50 build servers).