Simplify response type of FindLocalBranches

Probably for legacy reasons, the FindLocalBranches RPC does not use the Branch protobuf message in its response. This adds maintenance overhead, see recent bug fixes: !800 (merged) !822 (merged)

It would be better if we just used the Branch message in the response. When we change this we need to do so in a backwards-compatible manner, i.e. keep sending the old fields, until 1 gitlab release after this is fixed.

Relates to #1294 (closed)