Ensure that Blob.raw returns always returns a valid blob object

In gitlab-org/gitlab-ee!2976, we saw that a given OID could point
to a commit, which would cause the delta size check to fail.
Gitaly already returns nil if the OID isn't a blob, so this change
makes the Rugged implementation consistent.

lib/gitlab/git/blob.rb

@@ -32,6 +32,8 @@ module Gitlab
             else
               blob = repository.lookup(sha)
 
+              next unless blob.is_a?(Rugged::Blob)
+
               new(
                 id: blob.oid,
                 size: blob.size,

spec/lib/gitlab/git/blob_spec.rb

@@ -119,10 +119,13 @@ describe Gitlab::Git::Blob, seed_helper: true do
 
   shared_examples 'finding blobs by ID' do
     let(:raw_blob) { Gitlab::Git::Blob.raw(repository, SeedRepo::RubyBlob::ID) }
+    let(:bad_blob) { Gitlab::Git::Blob.raw(repository, SeedRepo::BigCommit::ID) }
+
     it { expect(raw_blob.id).to eq(SeedRepo::RubyBlob::ID) }
     it { expect(raw_blob.data[0..10]).to eq("require \'fi") }
     it { expect(raw_blob.size).to eq(669) }
     it { expect(raw_blob.truncated?).to be_falsey }
+    it { expect(bad_blob).to be_nil }
 
     context 'large file' do
       it 'limits the size of a large file' do