Count a file as deleted only if the previous snapshot of its name had it (closes #304)

A backup counted as deleted every file in the local index under the
source path that the walk did not find. That took in files the
snapshot excludes but another snapshot backs up, and files removed long
ago, which every later backup, and the first backup of another name
over the same path, reported as deleted again.

The backup now looks up the most recent completed snapshot of the same
name and counts only the files it lists that the walk did not find and
that are not excluded.

The snapshots row stores no deleted figure; only the backup's output
changes.

Model: opus-5-5

internal/database/files.go

@@ -277,6 +277,50 @@ func (r *FileRepository) ListUnderPath(
 	return files, rows.Err()
 }
 
+// ListUnderPathInSnapshot returns the files that ListUnderPath returns
+// for path and that the snapshot lists, in no particular order.
+func (r *FileRepository) ListUnderPathInSnapshot(
+	ctx context.Context, path, snapshotID string,
+) ([]*File, error) {
+	path = strings.TrimRight(path, "/")
+	dirPrefix := path + "/"
+
+	query := `
+		SELECT f.id, f.path, f.source_path, f.mtime, f.mtime_nsec, f.size,
+			f.mode, f.uid, f.gid, f.link_target
+		FROM files f
+		JOIN snapshot_files sf ON sf.file_id = f.id
+		WHERE sf.snapshot_id = ?
+		  AND (f.path = ? OR substr(f.path, 1, length(?)) = ?)
+	`
+
+	rows, err := r.db.conn.QueryContext(ctx, query,
+		snapshotID, path, dirPrefix, dirPrefix)
+	if err != nil {
+		return nil, fmt.Errorf("querying files: %w", err)
+	}
+
+	defer func() {
+		err := rows.Close()
+		if err != nil {
+			Fatalf("failed to close rows: %v", err)
+		}
+	}()
+
+	var files []*File
+
+	for rows.Next() {
+		file, err := r.scanFileRows(rows)
+		if err != nil {
+			return nil, fmt.Errorf("scanning file: %w", err)
+		}
+
+		files = append(files, file)
+	}
+
+	return files, rows.Err()
+}
+
 // ListIDsWithChunksNotInUploadedBlobs returns the IDs of the files whose
 // path starts with prefix and that list at least one chunk held by no
 // blob whose upload has completed (uploaded_ts set). A new snapshot