Check every group member and show the content phase from its start

The content phase now checks every record sharing a size, head and
tail with lstat, including those that already have a content hash, and
reads those without one only when at least two pass. Only a missing
file is passed over silently; any other lstat error is warned about and
counted as skipped. The phase shows a running count while it queries
and checks, then its bar. The README states once when content is empty.
New tests cover these and hard links.

Model: opus-5-5

db.go

@@ -279,31 +279,29 @@ func loadFileMeta(ctx context.Context, db *sql.DB,
 }
 
 // contentCandidatesSQL selects every record of at least headTailMin
-// bytes that has no content hash but whose size, head, and tail equal
-// another record's, with the number of records in its group (the
-// records sharing that size, head, and tail) that already have a
-// content hash. SQLite does the grouping, so no other record's hashes
-// are loaded into memory; the rows come ordered by size, head, and
-// tail, so each group's rows arrive together.
+// bytes whose size, head, and tail equal another record's, in each
+// group (the records sharing a size, head, and tail) where at least one
+// record has no content hash, with whether each record has one. SQLite
+// does the grouping, so no other record's hashes are loaded into
+// memory; the rows come ordered by size, head, and tail, so each
+// group's rows arrive together.
 const contentCandidatesSQL = `
-SELECT f.path, f.size, f.mtime, f.head, f.tail, g.hashed
+SELECT f.path, f.size, f.mtime, f.head, f.tail, f.content <> ''
 FROM files AS f
 JOIN (
-    SELECT size, head, tail, SUM(content <> '') AS hashed
+    SELECT size, head, tail
     FROM files
     WHERE size >= ? AND head <> ''
     GROUP BY size, head, tail
-    HAVING COUNT(*) > 1
+    HAVING COUNT(*) > 1 AND SUM(content = '') > 0
 ) AS g USING (size, head, tail)
-WHERE f.content = ''
 ORDER BY size, head, tail
 `
 
 // loadContentCandidates streams the rows of contentCandidatesSQL to fn:
-// each record, without its content hash, and the number of records in
-// its group that already have one.
+// each record, without its content hash, and whether it has one.
 func loadContentCandidates(ctx context.Context, db *sql.DB,
-	fn func(r scanRec, hashed int),
+	fn func(r scanRec, hashed bool),
 ) error {
 	rows, err := db.QueryContext(ctx, contentCandidatesSQL, headTailMin)
 	if err != nil {
@@ -316,7 +314,7 @@ func loadContentCandidates(ctx context.Context, db *sql.DB,
 		var (
 			path   []byte
 			r      scanRec
-			hashed int
+			hashed int64
 		)
 
 		err = rows.Scan(&path, &r.size, &r.mtime, &r.head, &r.tail, &hashed)
@@ -325,7 +323,7 @@ func loadContentCandidates(ctx context.Context, db *sql.DB,
 		}
 
 		r.path = string(path)
-		fn(r, hashed)
+		fn(r, hashed != 0)
 	}
 
 	err = rows.Err()