Don't recurse in local backend's List if not required

Due to the return if !isFile, the IsDir branch in List was never taken
and subdirectories were traversed recursively.

Also replaced isFile by an IsRegular check, which has been equivalent
since Go 1.12 (golang/go@a2a3dd00c9).

internal/backend/local/local.go

@@ -220,32 +220,27 @@ func (b *Local) Remove(ctx context.Context, h restic.Handle) error {
 	return fs.Remove(fn)
 }
 
-func isFile(fi os.FileInfo) bool {
-	return fi.Mode()&(os.ModeType|os.ModeCharDevice) == 0
-}
-
 // List runs fn for each file in the backend which has the type t. When an
 // error occurs (or fn returns an error), List stops and returns it.
 func (b *Local) List(ctx context.Context, t restic.FileType, fn func(restic.FileInfo) error) error {
 	debug.Log("List %v", t)
 
 	basedir, subdirs := b.Basedir(t)
+	atBasedir := true
 	err := fs.Walk(basedir, func(path string, fi os.FileInfo, err error) error {
 		debug.Log("walk on %v\n", path)
 		if err != nil {
 			return err
 		}
 
-		if path == basedir {
+		switch {
+		case atBasedir: // Skip basedir itself.
+			atBasedir = false
 			return nil
-		}
-
-		if !isFile(fi) {
-			return nil
-		}
-
-		if fi.IsDir() && !subdirs {
+		case fi.IsDir() && !subdirs:
 			return filepath.SkipDir
+		case !fi.Mode().IsRegular():
+			return nil
 		}
 
 		debug.Log("send %v\n", filepath.Base(path))