Steps to reproduce:

• Set up a reference repository in the build node's remote root directory under refrepo/my_repo.git
• Configure a new job to use this reference repo both during clone and submodule update:
• Start a build. Gives the following output:
   

  The recommended git tool is: NONE
  using credential ****
  Cloning the remote Git repository
  Cloning repository ssh://git@****/my_repo.git
   > git init /home/**** # timeout=10
  Using reference repository: refrepo/my_repo.git
  Fetching upstream changes from ssh://git@****/my_repo.git
   > git --version # timeout=10
   > git --version # 'git version 2.7.4'
  using GIT_SSH to set credentials SSH Key for accessing ****
   > git fetch --tags --progress ssh://git@****/my_repo.git +refs/heads/*:refs/remotes/origin/* # timeout=10
  Avoid second fetch
  Checking out Revision **** (main)
  Commit message: "****"
   > git config remote.origin.url ssh://git@****/my_repo.git # timeout=10
   > git config --add remote.origin.fetch +refs/heads/*:refs/remotes/origin/* # timeout=10
   > git config core.sparsecheckout # timeout=10
   > git checkout -f **** # timeout=10
   > git rev-list --no-walk **** # timeout=10
  hudson.plugins.git.GitException: Command "git submodule update --reference refrepo/my_repo.git modules/my_submodule" returned status code 128:
  stdout: 
  stderr: Cloning into 'modules/my_submodule'...
  fatal: Could not switch to 'refrepo/': No such file or directory
  fatal: clone of 'ssh://git@****/my_submodule.git' into submodule path 'modules/my_submodule' failed
  

I'm not sure if relative paths to reference repositories are supposed to work/supported in the Git client plugin, but I find this inconsistency between clone and submodule update odd.