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

db.go

@@ -11,6 +11,7 @@ import (
 	"path/filepath"
 	"slices"
 	"strconv"
+	"strings"
 	"time"
 
 	"golang.org/x/sys/unix"
@@ -91,13 +92,31 @@ var errSchemaVersion = errors.New("unsupported database schema version")
 var errScanRunning = errors.New("another scan is running")
 
 // databasePath resolves the database location: SFDUPES_DATABASE when
-// set and non-empty, the compiled-in default otherwise.
-func databasePath() string {
+// set and non-empty, the compiled-in default otherwise. It returns the
+// path, with the working directory put in front of it when it is
+// relative, and where the path came from, "default" or "from
+// SFDUPES_DATABASE", for the line each subcommand prints first on
+// stderr.
+func databasePath() (string, string, error) {
+	path, source := defaultDatabasePath, "default"
 	if p := os.Getenv(databaseEnv); p != "" {
-		return p
+		path, source = p, "from "+databaseEnv
 	}
 
-	return defaultDatabasePath
+	if filepath.IsAbs(path) {
+		return path, source, nil
+	}
+
+	wd, err := os.Getwd()
+	if err != nil {
+		return "", "", fmt.Errorf("resolve database path %s: %w", path, err)
+	}
+
+	// Not filepath.Abs or filepath.Join: they clean the path, which
+	// drops each ".." together with the directory name before it. When
+	// that directory is a symlink, the cleaned path names another file.
+	// The working directory ends in "/" only when it is the root.
+	return strings.TrimSuffix(wd, "/") + "/" + path, source, nil
 }
 
 // scanParams are the connection parameters for scan: read-write, with