Stale dynamic repo cache

[MattDavies]

In 3.0.0-beta0002 it looks like caching for the dynamic repository was added, but that cache is never updated over time and becomes stale. As an example when new tags are added in the remote, GitVersion doesn't seem to see them in it's cached copy of the repository and we needed to manually delete the dynamic repo cache first to get a correct version number result.

I'm guessing it's related to this: #399 (diff) but I haven't had a chance to investigate further yet.

[JakeGinnivan]

Hrrm, maybe normalise doesn't fetch.. damn