Sharding breaks sorting on gateway

#### Version information:
```
go-ipfs version: 0.4.13-
Repo version: 6
System version: amd64/linux
Golang version: go1.9.2
```

#### Type:
Bug

#### Severity:
Low
#### Description:

I have the following directory structrure:

```
.
├── a
├── b
├── c
└── f

4 directories, 0 files
```

If I add this with sharding disabled, I get this:

```
added QmUNLLsPACCz1vLxQVkXqqLX5R1X345qqfHbsf67hvA3Nn t/a
added QmUNLLsPACCz1vLxQVkXqqLX5R1X345qqfHbsf67hvA3Nn t/b
added QmUNLLsPACCz1vLxQVkXqqLX5R1X345qqfHbsf67hvA3Nn t/c
added QmUNLLsPACCz1vLxQVkXqqLX5R1X345qqfHbsf67hvA3Nn t/f
added QmZ2EQQs5XWXazqbnqXB67ZAhGzkEZzDTM9MY2R3mr9251 t
```

This is how it looks on the gateway:


If I activate sharding and add it again, I get this:

```
added QmUNLLsPACCz1vLxQVkXqqLX5R1X345qqfHbsf67hvA3Nn t/a
added QmUNLLsPACCz1vLxQVkXqqLX5R1X345qqfHbsf67hvA3Nn t/b
added QmUNLLsPACCz1vLxQVkXqqLX5R1X345qqfHbsf67hvA3Nn t/c
added QmUNLLsPACCz1vLxQVkXqqLX5R1X345qqfHbsf67hvA3Nn t/f
added QmaY2Hm1WfweGcsn2WuyFAUfPJqfFH9giMtZHezvPjNgHe t
```

And now the gateway looks like this:


Notice that the order changed from:

- a
- b
- c
- f

To

- b
- a
- c
- f