Use a file for the empty pack

The empty pack is the same for all of these tests, so we can generate it
once, store it in the repo, and avoid the complexity of shelling out to
git during the tests.

Here's how I generated the file:

  $ git pack-objects --revs --stdout > internal/integration/testdata/empty.pack </dev/null
  Total 0 (delta 0), reused 0 (delta 0), pack-reused 0

internal/integration/hiderefs_test.go

@@ -110,11 +110,8 @@ func TestHiderefsConfig(t *testing.T) {
 
 	// Send an empty pack, since we're using commits that are already in
 	// the repo.
-	packObjects := exec.CommandContext(ctx, "git", "-C", testRepo, "pack-objects", "--all-progress-implied", "--revs", "--stdout", "--thin", "--delta-base-offset", "--progress")
-	packObjects.Stderr = os.Stderr
-	pack, err := packObjects.StdoutPipe()
+	pack, err := os.Open("testdata/empty.pack")
 	require.NoError(t, err)
-	go packObjects.Run()
 	if _, err := io.Copy(srpIn, pack); err != nil {
 		t.Logf("error writing pack to spokes-receive-pack input: %v", err)
 	}

internal/integration/nosideband_test.go

@@ -72,11 +72,8 @@ func TestNoSideBand(t *testing.T) {
 
 	// Send an empty pack, since we're using commits that are already in
 	// the repo.
-	packObjects := exec.CommandContext(ctx, "git", "-C", testRepo, "pack-objects", "--all-progress-implied", "--revs", "--stdout", "--thin", "--delta-base-offset", "--progress")
-	packObjects.Stderr = os.Stderr
-	pack, err := packObjects.StdoutPipe()
+	pack, err := os.Open("testdata/empty.pack")
 	require.NoError(t, err)
-	go packObjects.Run()
 	if _, err := io.Copy(srpIn, pack); err != nil {
 		t.Logf("error writing pack to spokes-receive-pack input: %v", err)
 	}

internal/integration/pushoptions_test.go

@@ -71,11 +71,8 @@ func TestPushOptions(t *testing.T) {
 
 	// Send an empty pack, since we're using commits that are already in
 	// the repo.
-	packObjects := exec.CommandContext(ctx, "git", "-C", testRepo, "pack-objects", "--all-progress-implied", "--revs", "--stdout", "--thin", "--delta-base-offset", "--progress")
-	packObjects.Stderr = os.Stderr
-	pack, err := packObjects.StdoutPipe()
+	pack, err := os.Open("testdata/empty.pack")
 	require.NoError(t, err)
-	go packObjects.Run()
 	if _, err := io.Copy(srpIn, pack); err != nil {
 		t.Logf("error writing pack to spokes-receive-pack input: %v", err)
 	}