fs: Add interface and FS implementations

This adds two implementations of the new `FS` interface: One for the local
file system (`Local`) and one for a single file read from an
`io.Reader` (`Reader`).

internal/fs/stat_test.go

@@ -0,0 +1,31 @@
+package fs
+
+import (
+	"io/ioutil"
+	"path/filepath"
+	"testing"
+
+	restictest "github.com/restic/restic/internal/test"
+)
+
+func TestExtendedStat(t *testing.T) {
+	tempdir, cleanup := restictest.TempDir(t)
+	defer cleanup()
+
+	filename := filepath.Join(tempdir, "file")
+	err := ioutil.WriteFile(filename, []byte("foobar"), 0640)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	fi, err := Lstat(filename)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	extFI := ExtendedStat(fi)
+
+	if !extFI.ModTime.Equal(fi.ModTime()) {
+		t.Errorf("extFI.ModTime does not match, want %v, got %v", fi.ModTime(), extFI.ModTime)
+	}
+}