Specifying flat option in downloadSpec throws java.lang.NoClassDefFoundError: org/eclipse/jgit/util/StringUtils

When using the Pipeline API download with the flat option, the plugin throws the exception mentioned in the Summary. It doesn't matter what value you set flat to; its presence in the downloadSpec map triggers the error. E.g.,

artiSvr.download '''
    {
        "files": [
          {
              "pattern": "some-repo/**/cool-proj.jar",
              "target": "localBins/",
              "flat": "true"
          }
        ]
    }
'''

The same goes for the other boolean option, recursive.

I believe this is because, at least in the jenkinsci repo, GenericDownloadExecutor is importing jgit.util.StringUtils while the dependency isn't actually packaged up with the current plugin.

I have a sneaking suspicion HAP-823 may refactor away this bug, but am unsure.

FWIW another user reported this as well on SO.