Support mergetag

Support commits with the mergetag. The example here uses a gpg signed tag.

Example:

```bash
tree a9ac6c1e58bbdd7693e49ce34b32d9b0b53c0bcf
parent dce37589cfa5748900d05ab07ee2af5010866838
parent b2b1760347d797f3dc79360d487b9afa7baafd6a
author Jonathan Hoyt <hoyt@github.com> 1340923820 -0700
committer Jonathan Hoyt <hoyt@github.com> 1340923820 -0700
mergetag object b2b1760347d797f3dc79360d487b9afa7baafd6a
 type commit
 tag v1.1
 tagger Jonathan Hoyt <hoyt@github.com> 1340923695 -0700
 
 signed tag in foo
 -----BEGIN PGP SIGNATURE-----
 Version: GnuPG v1.4.12 (Darwin)
 
 iQEcBAABAgAGBQJP7N8vAAoJEM+49UOrepeVvk0IAMg8rB5eM0wFPC2nBy9ji4rp
 mM2xmrmEJmI7IKbY4TxtFTXq8kU9YhETBj5ZDTzUysPNDA5n+ZkZ/8MeXGTp4ml6
 kdDUIGXfAt7UXsEUjVw4C0Ln8TbxZEk2bDGWoEuPuMcwqj0aNihIcxb0lg7iYoq4
 THdKlbaA4qvF2Su42tayLOcT3pmIIZilQjSe6eilYLkqz9VpW0Fi/eMhe1WD2Vc7
 A97pq0garOYg38roQC9JsLyx5gBsaWix30IxtwIQdPZB77rRTZGoCfofOvZvwJtD
 4LQoMEzppxHMIdBbKyOaU5i/RjBxtJIScwlHXd9FSujn9ydAJdj8gvpyRtvCkCE=
 =yhJX
 -----END PGP SIGNATURE-----

Merge tag 'v1.1' into bar

signed tag in foo

* tag 'v1.1':
  foo
```