[Fix JENKINS-22039] Use new temp dir for remote polling credentials check

The default behavior was to create a new git directory in the system
temporary directory.  Since git repositories apply to all child
directories, that had the bad result of making the system temporary
directory appear as though it were under git control.

That also caused spurious automated test failures in the git-client
unit tests.
[maven-release-plugin] prepare release git-client-1.8.1
[maven-release-plugin] prepare for next development iteration