[FIXED JENKINS-19458] Intern CvsFile instances

[JENKINS-19458](https://issues.jenkins-ci.org/browse/JENKINS-19458)

A user of the plugin working with a CVS repository with ~200k files, but many jobs using this repository, reported ever-increasing heap usage, in the 2.5Gb vicinity. A heap histogram showed 10M instances of `CvsFile` held by `CvsRevisionState`. After running with this patch, memory usage is considerably better (~1.5Gb) and the count of `CvsFile`s holds steady at around ~200k.