Remove FindCommit calls from raw endpoint

In #661 (closed), we found that many concurrent calls to the raw blob endpoint using a ref, not a SHA, could cause Gitaly's FindCommit method some pain.

In #661 (comment 450983566), Bob pointed out we don't need to call FindCommit at all for this action:

We could probably also remove the FindCommit from that endpoint.

The @commit.id that we're using in RawController#show isn't really needed, we can also pass a ref-name to blob_at.

There's some validation that happens in ExtractsPath#assign_ref_vars but I wonder if we could optimize there by only triggering the blob-rpc, since we don't really need any of the other assigned instance variables.