Say "skipped" in scan's warnings for skipped paths (closes #170)

A warning for a path the summary counts as skipped now reads
`scan: skipped PATH (REASON)`, naming the path once. This covers
unreadable directories, failed lstats, failed reads and dropped
operands. Each path of a hard-linked file that cannot be read gets its
own line, since each is counted. Lines for paths left out on purpose
(`--exclude`, default excludes, filesystems that are not persistent)
now read `scan: left out PATH (...)`, so a search for `scan: skipped`
finds only the counted paths.

Model: opus-5-5

README.md

@@ -689,10 +689,11 @@ scan: skipped /srv/a/closed (permission denied)
 scan: skipped /srv/a/pipe (a FIFO)
 ```
 
-The summary counts each path named in such a warning. A line for a path left out
-on purpose starts with `scan: left out` instead, and is not counted. A search of
-the log for `scan: skipped` therefore finds the paths the summary counts. When a
-file with several hard links cannot be read, each of its paths gets a warning.
+The summary counts each path named in such a warning. When a file with several
+hard links cannot be read, each of its paths gets a warning. A line for a path
+left out on purpose starts with `scan: left out` instead, and is not counted. A
+search of the log for `scan: skipped` therefore finds the paths the summary
+counts.
 
 #### Stopping a scan
 

internal/sfdupes/scan_test.go

@@ -752,9 +752,8 @@ func TestWalkPassesOverRemovedPaths(t *testing.T) {
 
 // TestScanWarnsAboutUnreadablePaths checks that a permission error is
 // still warned about and counted as skipped, unlike a removed path. The
-// walk reaches an unreadable directory. It also reaches a file and a
-// subdirectory in a directory that can be read but not searched. Another
-// such file is an operand.
+// walk reaches an unreadable directory, and a file in a directory that
+// can be read but not searched. Another such file is an operand.
 //
 //nolint:paralleltest // captureStderr replaces the process-wide os.Stderr
 func TestScanWarnsAboutUnreadablePaths(t *testing.T) {
@@ -766,7 +765,6 @@ func TestScanWarnsAboutUnreadablePaths(t *testing.T) {
 	dir := t.TempDir()
 	tree := filepath.Join(dir, "tree")
 	walked := writeFile(t, tree, "unsearchable/f", []byte("x"))
-	subdir := filepath.Join(filepath.Dir(walked), "d")
 	operand := writeFile(t, dir, "unsearchable/f", []byte("y"))
 	unreadable := filepath.Join(tree, "unreadable")
 
@@ -775,11 +773,6 @@ func TestScanWarnsAboutUnreadablePaths(t *testing.T) {
 		t.Fatal(err)
 	}
 
-	err = os.Mkdir(subdir, 0o750)
-	if err != nil {
-		t.Fatal(err)
-	}
-
 	modes := map[string]fs.FileMode{
 		unreadable:            0,
 		filepath.Dir(walked):  0o400,
@@ -800,15 +793,14 @@ func TestScanWarnsAboutUnreadablePaths(t *testing.T) {
 	}
 
 	st := syncTree(t, openTestDB(t), tree, operand)
-	if st != (scanStats{skipped: 4}) {
-		t.Errorf("stats = %+v, want 4 skipped and nothing else", st)
+	if st != (scanStats{skipped: 3}) {
+		t.Errorf("stats = %+v, want 3 skipped and nothing else", st)
 	}
 
 	got := stderr()
 	for _, warning := range []string{
 		"scan: skipped " + unreadable + " (permission denied)\n",
 		"scan: skipped " + walked + " (permission denied)\n",
-		"scan: skipped " + subdir + " (permission denied)\n",
 		"scan: skipped " + operand + " (permission denied)\n",
 	} {
 		if !strings.Contains(got, warning) {