RepositoryService: implement FindMergeBase RPC

After spending some time with Gitaly and Rails side, found that it's all about returning the gca (greatest common ancestor) of two revisions, which will be used as Base in a 3-way merge.

Not very important I guess but, a Note as per Gitaly: FindMergeBaseRequest can accept more than two revisions (but that's preserved for the future so that later we don't require to make changes in the proto file) in the cases of octopus merges.

Closes #26 (closed)

Edited by Sushil Khanchi

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