JENKINS-20531

A fix for https://issues.jenkins-ci.org/browse/JENKINS-20531

Catches the GitException and throws an AbortException instead, using the same pattern as 10 lines above.
Good point. I believe the original intent of the change was to follow the pattern used about 10 lines earlier in the file.  However, I like the AbortException wrapping the GitException even better, since that avoids a stack trace being printed, and still carries the information along.