hdfs: fix incorrect modtime after uploading a file or setting its modtime

SetModTime (which Put and Update also use) kept the caller's full
precision modtime in memory while Chtimes sets it with second
precision, so until the object was re-read the modtime did not match
what a fresh listing would report. This broke wrappers which compare
exact modtimes, such as the hasher backend's fingerprint and the VFS
cache. Truncate the modtime to second precision to match what the
server stores.

backend/hdfs/object.go

@@ -50,7 +50,10 @@ func (o *Object) SetModTime(ctx context.Context, modTime time.Time) error {
 	if err != nil {
 		return err
 	}
-	o.modTime = modTime
+	// Chtimes sets the modtime with second precision so truncate
+	// here too to keep the in-memory modtime identical to the one a
+	// fresh listing returns.
+	o.modTime = modTime.Truncate(time.Second)
 	return nil
 }