Namespace support for multiple branches

[ttgirod]

As already mentioned in #122, currently the only way to address multiple branch indexing in the same repository is making the branch an explicit part of the repository name. Adding a new repository with the same name but a different branch would silently overwrite an already existing entry with the same repository name.

It would actually be good to have some more convenient way to accommodate repository + branches name space.

[ttgirod]

Quoting @boyter's comment on this from #122:

Having better support for branches is one of those things I have in the TODO pile. I would like to allow adding a repository and then it would be inspected for all branches and you could then pick which ones to index. I need to have a think about the implications that has for adding via API and the like.
It may mean I will do a hard break with the current way this is implemented to allow this.