Record file mode in the manifest, 0000 unless asked (closes #161)

MFFilePath gains mode (field 304): a file's permission bits, 0777 at
most, or 0000, meaning none recorded. gen and freshen record real modes
only with --include-permissions (ScannerOptions.IncludePermissions); the
builder keeps only mode.Perm(), so setuid, setgid and sticky are never
written. list -l and export show the mode in octal. check reports
MODE_MISMATCH for a recorded mode other than 0000 the file lacks. fetch
refuses a manifest with a mode above 0777 before requesting any file,
sets only the permission bits of each recorded mode on the files it
writes, and downloads again a present file whose mode differs. The decoding-cost bound counts a file
entry at 176 bytes, up from 160.

Model: opus-5-5

internal/cli/freshen_test.go

@@ -99,6 +99,42 @@ func TestFreshenUnchanged(t *testing.T) {
 	}
 }
 
+// TestFreshenRecordsModeOnlyWhenAsked changes only the modes of a tree
+// after gen made its manifest, which recorded every mode as 0000. freshen
+// --include-permissions must record each file's permission bits, and a
+// later freshen without it must record 0000 again, although no file's
+// content or mtime changed.
+func TestFreshenRecordsModeOnlyWhenAsked(t *testing.T) {
+	t.Parallel()
+
+	fs := afero.NewOsFs()
+	root, manifestPath := setupFreshenDir(t, fs,
+		map[string]string{testFileTxt: "a file", testDirFile: "a file in dir"})
+	require.NoError(t, fs.Chmod(filepath.Join(root, testFileTxt), 0o640))
+	require.NoError(t, fs.Chmod(filepath.Join(root, testDirFile), 0o755))
+
+	recordedModes := func() map[string]uint32 {
+		modes := map[string]uint32{}
+		for _, f := range manifestFiles(t, fs, manifestPath) {
+			modes[f.GetPath()] = f.GetMode()
+		}
+
+		return modes
+	}
+
+	opts := testOpts([]string{
+		testApp, cmdFreshen, "-q", "--" + flagIncludePermissions,
+		testFlagBase, root, manifestPath,
+	}, fs)
+	require.Equal(t, 0, runCLI(opts), "stderr: %s", testStderr(t, opts))
+	assert.Equal(t, map[string]uint32{testFileTxt: 0o640, testDirFile: 0o755},
+		recordedModes())
+
+	runFreshen(t, fs, root, manifestPath)
+	assert.Equal(t, map[string]uint32{testFileTxt: 0, testDirFile: 0},
+		recordedModes())
+}
+
 // assertManifestLists asserts that the manifest at manifestPath lists
 // exactly the files in want, each with the size and SHA-256 hash of its
 // content in want and the mtime of the file of that name under root.