Name the database each command uses on stderr (closes #111)

scan, report and trees now print one line on stderr before anything
else, naming the database and whether its path is the default or comes
from SFDUPES_DATABASE; scan also says when the file is new. report and
trees print it before opening the database, so a missing-database error
follows it. A relative path gets the working directory put in front of
it without cleaning: cleaning drops a ".." with the directory before
it, which names another file when that directory is a symlink. README
"Database" lists when scan writes each file.

Unverified: the line on a terminal has no test of its own.
Rule suppressed: TestDatabasePath carries nolint:paralleltest, as it
changes the working directory.

Model: opus-5-5

trees.go

@@ -29,10 +29,16 @@ type treeNode struct {
 // the record paths, computes a Merkle-style digest per directory, and
 // prints maximal duplicate-tree groups as TSV on stdout. It never
 // touches the scanned filesystem; its only I/O is the database, stdout,
-// and stderr. Any database problem, including a missing database, is
-// fatal.
+// and stderr. It names the database on stderr before it opens it. Any
+// database problem, including a missing database, is fatal.
 func runTrees(ctx context.Context, stdout io.Writer) error {
-	dbPath := databasePath()
+	dbPath, source, err := databasePath()
+	if err != nil {
+		return err
+	}
+
+	fmt.Fprintf(os.Stderr, "trees: database %s (%s)\n",
+		escapePath(dbPath), source)
 
 	db, err := openReportDatabase(ctx, dbPath)
 	if err != nil {