Merge branch 'jt/fetch-allow-tip-sha1-implicitly'

There is no good reason why "git fetch $there $sha1" should fail when the $sha1 names an object at the tip of an advertised ref, even when the other side hasn't enabled allowTipSHA1InWant. * jt/fetch-allow-tip-sha1-implicitly: fetch-pack: always allow fetching of literal SHA1s