Merge branch 'jc/fetch-raw-sha1'

Allows requests to fetch objects at any tip of refs (including hidden ones). It seems that there may be use cases even outside Gerrit (e.g. $gmane/215701). * jc/fetch-raw-sha1: fetch: fetch objects by their exact SHA-1 object names upload-pack: optionally allow fetching from the tips of hidden refs fetch: use struct ref to represent refs to be fetched parse_fetch_refspec(): clarify the codeflow a bit