Support Google's repo

Google's source tree is managed via multiple git project using a tool called repo. More information about it can be found here: http://source.android.com/source/downloading.html

It would be desirable for FishEye to support this, whereas there are multiple git repositories and make them searchable as if it is one single git repository (in reality there are multiple git repositories). FishEye should be able to index all repositories in one manifest as one single repository and provide all the features accordingly.