I have just discovered that if we configure the Jenkins job to contain wildcards in the Branch Specifier field, e.g. origin/personal/**, then the polling of the job would just hang after the first line:
Git Polling Log
Started on Oct 24, 2014 4:37:58 PM
This would stay like that until the currently running build of this job completes. Then polling would go through in sub-seconds. I tried specifying a fully spelt-out branch specifier, and this problem goes away.
Normally this would not be a problem, but we are now trying to enable concurrent builds for this job. That means, as long as there's one build in progress for this job, no additional builds can start (since polling is hanging) until the current build finishes. So this bug effectively breaks concurrent jobs that relies on polling.
We are on Jenkins 1.578, and Git plugin 2.2.7.