Bump vendored libgit2 to fba70a9d5f

This includes updating the `Index.WriteTreeTo` test as it was abusing an
oversight of the object creation safety checks and creating a tree referencing
a non-existent blob. Instead we update it to the primary purpose of this method
which is to write into a repository from an unattached index.

index_test.go

@@ -3,6 +3,7 @@ package git
 import (
 	"io/ioutil"
 	"os"
+	"path"
 	"runtime"
 	"testing"
 )
@@ -59,14 +60,29 @@ func TestIndexWriteTreeTo(t *testing.T) {
 	repo := createTestRepo(t)
 	defer cleanupTestRepo(t, repo)
 
-	repo2 := createTestRepo(t)
-	defer cleanupTestRepo(t, repo2)
+	idx, err := NewIndex()
+	checkFatal(t, err)
 
-	idx, err := repo.Index()
+	odb, err := repo.Odb()
 	checkFatal(t, err)
-	err = idx.AddByPath("README")
+
+	content, err := ioutil.ReadFile(path.Join(repo.Workdir(), "README"))
 	checkFatal(t, err)
-	treeId, err := idx.WriteTreeTo(repo2)
+
+	id, err := odb.Write(content, ObjectBlob)
+	checkFatal(t, err)
+
+	err = idx.Add(&IndexEntry{
+		Mode: FilemodeBlob,
+		Uid:  0,
+		Gid:  0,
+		Size: uint32(len(content)),
+		Id:   id,
+		Path: "README",
+	})
+	checkFatal(t, err)
+
+	treeId, err := idx.WriteTreeTo(repo)
 	checkFatal(t, err)
 
 	if treeId.String() != "b7119b11e8ef7a1a5a34d3ac87f5b075228ac81e" {

vendor/libgit2

@@ -1 +1 @@
-Subproject commit 838a2f2918b6d9fad8768d2498575ff5d75c35f0
+Subproject commit fba70a9d5f1fa433968a3dfd51e3153c8eebe834