MercurialRepositoryService, initial definition
This is the first definition of Mercurial (or Heptapod) gRPC methods. It takes the form of a dedicated service, in a separate `hgitaly` protobuf package. ## About the separate package The protobufs `package` directive translates directly into a the module name in Ruby generated code. This doesn't happen with Python because Python modules and packages names are derived from the file system layout, wich is obviously out of control of the code generation. Upstream's Ruby generated code is packaged in a separate gem, historically from the `gitaly-proto` project, and soon to be built from the bundled proto files within Gitaly. We are not not going to fork either of those, and hence we need our own package. At some point, we may also need to generate a Go library, which require a `option go_package` statement that we couldn't provide right away because it means choosing the appropriate full path (see examples in upstream GitLab proto files). A consequence of u...