Fix nil tx bug

[alcortesm]

Commit 5a72b12 introduced a bug that makes go-git panic when cloning repositories from non-seekable packfiles (from ssh or http connections for example). A simple way to reproduce the bug is:

package main

import (
	"log"

	git "gopkg.in/src-d/go-git.v4"
)

func main() {
	r := git.NewMemoryRepository()

	opts := &git.CloneOptions{
		URL: "[email protected]:src-d/git-fixture.git",
	}

	if err := r.Clone(opts); err != nil {
		log.Fatal(err)
	}
}

The bug has not been detected by our tests because some of the functionality being added by the commit was not being tested.

This fix has two commits: one that adds a test for the offending functionality and another to fix the bug.