Mercurial backend: repo should be private attribute

Hey, we agreed to make underlying backends repo attribute private (_repo ;-)). I suspect that some codes may be affected by this (even if external apps should never use underlying hg repo or dulwich repo directly). Marcin, is that the case? Can we change this attribute to _repo at MercurialRepository?

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Reference: https://bitbucket.org/marcinkuzminski/vcs/issues/42/
