Add verify-pack command

[pjbgf]

The core motivation to introduce this command is to make it easier to debug go-git tests that rely on go-git-fixtures. Now it is possible to quickly check a fixture's packfile by URL or tag:

$ gogit verify-pack --verbose --fixture-url https://github.com/git-fixtures/basic.git
e8d3ffab552895c19b9fcf7aa264d277cde33881 commit 254 174 12
6ecf0ef2c2dffb796033e5a02219af86ec6584e5 commit 93 100 186 1 e8d3ffab552895c19b9fcf7aa264d277cde33881
918c48b83bd081e863dbe1b80f8998f058cd8294 commit 242 163 286
af2d6a6954d532f8ffb47615169c8fdf9d383a1a commit 242 166 449
1669dce138d9b841a518c64b10914d88f5e488ea commit 333 223 615
a5b8b09e2f8fcb0bb99d3ccb0958157b40890d69 commit 332 225 838
35e85108805c84807bc66a02d91535e1e24b38b9 commit 244 167 1063
b8e471f58bcbca63b07bda20e428190409c2db47 commit 243 162 1230
b029517f6300c2da0f4b651b8642506cd6aaf45d commit 187 132 1392
32858aad3c383ed1ff0a0f9bdf231d54a00c9e88 blob   189 161 1524
d3ff53e0564a9f87d8e84b6e28e5060e517008aa blob   18 28 1685
c192bd6a24ea1ab01d78686e417c8bdc7c3d197f blob   1072 638 1713
d5c0f4ab811897cadf03aec358ae60d21f91c50d blob   76110 75699 2351
880cd14280f4b9b6ed3986d6671f907d7cc2a198 blob   2780 832 78050
49c6bb89b17060d7b4deacb7b338fcc6ea2352a9 blob   217848 1843 78882
c8f1d8c61f9da76f4cb49fd86322b6e685dba956 blob   706 273 80725
9a48f23120e880dfbe41f7c9b7b708e9ee62a492 blob   11488 3034 80998
9dea2395f5403188298c1dabe8bdafe562c491e3 blob   78 83 84032
dbd3641b371024f44d0e469a9c8f5457b0660de1 tree   272 260 84115
a8d315b2b1c615d43042c3a62402b8a54288cf5c tree   43 55 84375 1 dbd3641b371024f44d0e469a9c8f5457b0660de1
a39771a7651f97faf5c72e08224d857fc35133db tree   38 49 84430
5a877e6a906a2743ad6e45d99c1793642aaf8eda tree   75 80 84479
586af567d0bb5e771e49bdd9434f5e0fb76d25fa tree   38 49 84559
cf4aa3b38974fb7d81f367c0830f7d78d65ab86b tree   34 45 84608
7e59600739c96546163833214c36459e324bad0a blob   9 18 84653
fb72698cab7617ac416264415f13224dfd7a165e tree   6 17 84671 2 a8d315b2b1c615d43042c3a62402b8a54288cf5c
4d081c50e250fa32ea8b1313cf8bb7c2ad7627fd tree   9 20 84688 2 a8d315b2b1c615d43042c3a62402b8a54288cf5c
eba74343e2f15d62adedfd8c883ee0262b5c8021 tree   6 17 84708 2 a8d315b2b1c615d43042c3a62402b8a54288cf5c
c2d30fa8ef288618f65f6eed6e168e0d514886f4 tree   5 16 84725 1 dbd3641b371024f44d0e469a9c8f5457b0660de1
8dcef98b1d52143e1e2dbc458ffe38f925786bf2 tree   8 19 84741 2 a8d315b2b1c615d43042c3a62402b8a54288cf5c
aa9b383c260e1d05fbbf6b30a02914555e20c725 tree   4 14 84760 3 8dcef98b1d52143e1e2dbc458ffe38f925786bf2
non delta: 23 objects
chain length = 1: 3 objects
chain length = 2: 4 objects
chain length = 3: 1 object
https://github.com/git-fixtures/basic.git: ok