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

scan.go

@@ -91,9 +91,11 @@ type fileMeta struct {
 // least one other file shares are ever hashed: a size-unique file
 // cannot be a duplicate. A file of headTailMin or more gets its content
 // hash only when its size, head, and tail match another file's. Flag
-// parsing and the at-least-one-operand check are done by cobra. The
-// scan holds the lock on the database for its whole run, so a second
-// scan fails before it walks the filesystem or opens the database.
+// parsing and the at-least-one-operand check are done by cobra. Before
+// anything else, the scan names the database on stderr, saying whether
+// it is new. The scan holds the lock on the database for its whole
+// run, so a second scan fails before it walks the filesystem or opens
+// the database.
 // Errors are returned rather than exiting, so that the deferred close —
 // which takes the database out of WAL mode — always runs, and the lock
 // is released after it. When ctx is cancelled, as by the SIGINT or
@@ -104,12 +106,26 @@ type fileMeta struct {
 func runScan(ctx context.Context, roots []string, workers int,
 	oneFS bool,
 ) error {
-	roots, err := resolveRoots(roots)
+	dbPath, source, err := databasePath()
 	if err != nil {
 		return err
 	}
 
-	dbPath := databasePath()
+	// A database file missing now is created by lockScanDatabase below.
+	what := "database"
+
+	_, err = os.Stat(dbPath)
+	if errors.Is(err, fs.ErrNotExist) {
+		what = "new database"
+	}
+
+	fmt.Fprintf(os.Stderr, "scan: %s %s (%s)\n", what, escapePath(dbPath),
+		source)
+
+	roots, err = resolveRoots(roots)
+	if err != nil {
+		return err
+	}
 
 	lock, err := lockScanDatabase(dbPath)
 	if err != nil {