Ivan Nikitin

`Rugged::SubmoduleCollection#clone_submodule` assumes that [**every** repository has `master` branch](https://github.com/libgit2/rugged/blob/c5ed931dfc34fc1c8990d86c565492f610014c00/lib/rugged/submodule_collection.rb#L47). It is usually true but not generally correct. For instance [xcode-test-engine for arc](https://github.com/google/arc-xcode-test-engine) has only `stable` and `develop`, last one is default....