Update the Gerrit upgrade job to check for new on disk h2 cache files.
We discovered well after the fact that Gerrit 3.5 added new (large)
cache files to disk that would've been good to be aware of prior to the
upgrade. This change will check for new files and produce a message if
they exist.
Change-Id: I4b52f95dd4b23636c0360c9960d84bbed1a5b2d4