Refuse a file entry with no hash when loading a manifest (closes #194)

Loading accepted a file entry whose hashes list was empty. check and
fetch then failed the file as a hash mismatch, and freshen left it out
of the rewritten manifest without a word.

NewManifestFromReader now refuses such an entry, naming its path, after
checking the path itself. freshen's addExistingToBuilder returns an
error for an entry with no hash instead of skipping it. The file entry
table in docs/FORMAT.md says readers reject an entry with none.

Model: opus-5-5

mfer/deserialize_path_test.go

@@ -165,6 +165,24 @@ func TestDeserializeRefusesPathListedTwice(t *testing.T) {
 	}
 }
 
+// A manifest holding a file entry with no hash is refused as it is loaded,
+// naming the path: there is nothing to check the file against.
+func TestDeserializeRefusesEntryWithNoHash(t *testing.T) {
+	t.Parallel()
+
+	id := uuid.NewV4()
+	inner, err := proto.Marshal(&MFFile{
+		Version: MFFile_VERSION_ONE,
+		Files:   []*MFFilePath{{Path: "x", Size: 1}},
+		Uuid:    id[:],
+	})
+	require.NoError(t, err)
+
+	_, err = NewManifestFromReader(bytes.NewReader(wrapInner(t, id, inner)))
+	require.ErrorIs(t, err, errEntryMissingHash)
+	require.EqualError(t, err, `file entry has no hash: "x"`)
+}
+
 // Entries of a path, an empty hash, an empty MIME type and empty modification
 // and change times are counted at 432 bytes each (176 + 112 + 16 + 64 + 64)
 // and take 16 bytes plus the path to encode. A 37-character path makes that
@@ -229,6 +247,8 @@ func TestDeserializeDropsUnknownFields(t *testing.T) {
 
 	entry := protowire.AppendTag(nil, 1, protowire.BytesType) // MFFilePath.path
 	entry = protowire.AppendString(entry, "a")
+	entry = protowire.AppendTag(entry, 3, protowire.BytesType) // MFFilePath.hashes
+	entry = protowire.AppendBytes(entry, nil)
 	entry = append(entry, unknown...)
 
 	id := uuid.NewV4()