v2.0.0: CommitIter panics when Close is called

[toqueteos]

If we add the following test to repository_test.go we can see this in action:

func (s *SuiteRepository) TestCommitIterClosePanic(c *C) {
    r, err := NewRepository(RepositoryFixture)
    r.Remotes["origin"].upSrv = &MockGitUploadPackService{}

    c.Assert(err, IsNil)
    c.Assert(r.Pull("origin", "refs/heads/master"), IsNil)

    commits := r.Commits()
    commits.Close()
}

This is due to https://github.com/src-d/go-git/blob/v2.0.0/repository.go#L98, it seems commit iteration needs a small rework.

Possible solutions

• Make iter.Add and iter.Close private. Consecuences: We can't stop iterators anymore unless they reach the end.
• Rework how iterators work all over the repo.
• Add a check on Close to avoid more than once calls to it.