Add a test for blobs

This has been in my worktree unadded for a while.

blob_test.go

@@ -0,0 +1,18 @@
+package git
+
+import (
+	"os"
+	"testing"
+)
+
+func TestCreateBlobFromBuffer(t *testing.T) {
+	repo := createTestRepo(t)
+	defer os.RemoveAll(repo.Workdir())
+
+	id, err := repo.CreateBlobFromBuffer(make([]byte, 0))
+	checkFatal(t, err)
+
+	if id.String() != "e69de29bb2d1d6434b8b29ae775ad8c2e48c5391" {
+		t.Fatal("Empty buffer did not deliver empty blob id")
+	}
+}