Changeset should bust cache for all branch names

[sandlerr]

Tags and commit hashes should be stable but branch tips are not. At the moment we only bust for master thanks to https://github.com/zendesk/samson/pull/511.

[v9n]

@sandlerr @grosser Do you guy think it's good if we git branch on local cached repo to see if the entered reference is a branch? This may helps us busting cache. Though I think that in practice prolly master is only thing that we care about.

I can put up a PR for this.

[grosser]

I think we opted to make it simple to reduce the number of api requests ... ideally we would get the sha and then cache on that without having to do api request ... but the we have to fetch the repo first :(

maybe we can hand the deploy object into the method and cache based on it ?