[FIXED JENKINS-21464] save current Build in BuildData before re-scheduling job
Avoid NPE in GitPublisher when slave is disconnected

Null pointer exception was reported in jenkins log file after the node
which hosted the preceding build was disconnected.
Checkout a known tag from the repo so test job doesn't fail prematurely
Noting that PathRestriction implies DisableRemotePoll.
Temporarily remove NPE test - fails in Jenkins, works from cmdline and IDE
Add a `GIT_PREVIOUS_SUCCESSFUL_COMMIT` environment variable
Test the environment variables