Add []byte to repo.LoadAndDecrypt and utils.LoadAll

This commit changes the signatures for repository.LoadAndDecrypt and
utils.LoadAll to allow passing in a []byte as the buffer to use. This
buffer is enlarged as needed, and returned back to the caller for
further use.

In later commits, this allows reducing allocations by reusing a buffer
for multiple calls, e.g. in a worker function.

internal/cache/backend_test.go

@@ -17,7 +17,7 @@ import (
 )
 
 func loadAndCompare(t testing.TB, be restic.Backend, h restic.Handle, data []byte) {
-	buf, err := backend.LoadAll(context.TODO(), be, h)
+	buf, err := backend.LoadAll(context.TODO(), nil, be, h)
 	if err != nil {
 		t.Fatal(err)
 	}
@@ -147,7 +147,7 @@ func TestErrorBackend(t *testing.T) {
 	loadTest := func(wg *sync.WaitGroup, be restic.Backend) {
 		defer wg.Done()
 
-		buf, err := backend.LoadAll(context.TODO(), be, h)
+		buf, err := backend.LoadAll(context.TODO(), nil, be, h)
 		if err == testErr {
 			return
 		}