git client plugin creates unlimited directories in /tmp/ for polling the scm

For every polling cycle the git client creates a new directory for storing the credentials in /tmp/ but forgets to delete the directory itself after finishing the polling.

This leads to reaching the system limit of maximum numbers of subdirectories after some time which will break the polling cycle.

• This results in jenkins not starting builds on scm changes.
• The normal build process is not affected.
• This could affect some other functionality using the tmp directory and creating files there.

This behaviour was introduced with JENKINS-22039.