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, with its symlinks resolved as the kernel reports it, and is not
cleaned: a ".." after a symlink names another file than the cleaned
path does. README "Database" lists when scan writes each file.

Unverified: the line on a terminal has no test of its own.

Model: opus-5-5

db_test.go

@@ -43,16 +43,47 @@ func openTestDB(t *testing.T) *sql.DB {
 }
 
 func TestDatabasePath(t *testing.T) {
-	t.Setenv(databaseEnv, "")
-
-	if got := databasePath(); got != defaultDatabasePath {
-		t.Errorf("databasePath() = %q, want %q", got, defaultDatabasePath)
+	// README §Database: SFDUPES_DATABASE when set and non-empty, the
+	// default otherwise, with the working directory, its symlinks
+	// resolved, put in front of a relative path. Nothing is cleaned, so
+	// a ".." stays.
+	dir, err := filepath.EvalSymlinks(t.TempDir())
+	if err != nil {
+		t.Fatal(err)
 	}
 
-	t.Setenv(databaseEnv, "/custom/place.sqlite")
+	t.Chdir(dir)
 
-	if got := databasePath(); got != "/custom/place.sqlite" {
-		t.Errorf("databasePath() = %q, want the env override", got)
+	fromEnv := "from " + databaseEnv
+	cases := []struct {
+		env, path, source string
+	}{
+		{"", defaultDatabasePath, "default"},
+		{"/custom/place.sqlite", "/custom/place.sqlite", fromEnv},
+		{"/custom/link/../place.sqlite", "/custom/link/../place.sqlite",
+			fromEnv},
+		{"sub/place.sqlite", dir + "/sub/place.sqlite", fromEnv},
+		{"link/../place.sqlite", dir + "/link/../place.sqlite", fromEnv},
+	}
+
+	for _, tc := range cases {
+		t.Setenv(databaseEnv, tc.env)
+
+		path, source, err := databasePath()
+		if err != nil || path != tc.path || source != tc.source {
+			t.Errorf("databasePath() with %s=%q = %q, %q, %v; want %q, %q",
+				databaseEnv, tc.env, path, source, err, tc.path, tc.source)
+		}
+	}
+
+	// In the root directory the path gets one slash in front, not two.
+	t.Chdir("/")
+	t.Setenv(databaseEnv, "place.sqlite")
+
+	path, _, err := databasePath()
+	if err != nil || path != "/place.sqlite" {
+		t.Errorf("databasePath() in / = %q, %v; want %q",
+			path, err, "/place.sqlite")
 	}
 }