Gitlab::Workhorse: dispatch blobs to HGitaly when appropriate

This time, we need a knob to call HGitaly only for fully native
projects: `GetBlobRequest` works by oid and `send_git_blob` takes
a Blob object as argument whose id is really a Git oid in the
HGitaly1 case, and of course a HGitaly oid in the fully native case.

In the HGitaly1 case, it may be frightening that the public side of
the call can have a Mercurial SHA (navigation trough a commit instead
of a branch). Yet internally it is correctly resolved to a Git
blob oid, but that is with hindsight not surprising: it boils down
to the fact that the HGitaly1 mode works, as this raw download system
is obviously by far not the only use case for blobs in GitLab.

branch : heptapod-stable
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