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. A
file that is excluded, or inside an excluded directory, is not counted.

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

Model: opus-5-5

internal/snapshot/scanner.go

@@ -1997,7 +1997,7 @@ func (s *Scanner) detectDeletedFiles(
 
 // excludedOrInExcludedDir reports whether filePath, which is at or
 // beneath rootPath, or any directory between rootPath and it, matches an
-// exclude pattern. A pattern such as **/node_modules matches the
+// exclude pattern. A pattern such as */.local/share/Trash matches the
 // directory but not the files inside it, which the walk skips too.
 func (s *Scanner) excludedOrInExcludedDir(filePath, rootPath string) bool {
 	for p := filePath; len(p) > len(rootPath); p = filepath.Dir(p) {

internal/vaultik/snapshot_deleted_count_test.go

@@ -121,14 +121,14 @@ func TestBackupDoesNotCountFileInExcludedDirectoryAsDeleted(t *testing.T) {
 	t.Parallel()
 
 	env := newDeletedCountEnv(t, map[string][]byte{
-		"top.txt":                 []byte("top"),
-		"app/node_modules/dep.js": []byte("dep"),
+		"top.txt":                         []byte("top"),
+		"user/.local/share/Trash/old.txt": []byte("old"),
 	})
 	env.setSnapshot("home", "")
 
 	assert.Contains(t, env.backUp(t, "home"), filesLine(2, 2, 0))
 
-	env.setSnapshot("home", "", "**/node_modules")
+	env.setSnapshot("home", "", "*/.local/share/Trash")
 	assert.Contains(t, env.backUp(t, "home"), filesLine(1, 0, 1))
 }