void UpsertHead(Commit commit)
{
var headBlob = headsContainer.GetBlockBlobReference(commit.SourceId.ToString());
var headBytes = commit.Id.ToByteArray();
headBlob.UploadFromByteArray(headBytes, 0, headBytes.Length, commit.AccessCondition());
commit.SourceETag = headBlob.Properties.ETag;
}