fix: compute IsTruncated from the absolute index in ListMultipartUploads

With an upload-id-marker the lister ranges over Uploads[startIndex:],
but passed the index of that subslice to hasMoreAfter, which indexes
the full list. A page that ended on the last remaining upload was still
reported as truncated, with next markers pointing at that upload, so a
client had to make one more request to get an empty page.

Iterate over the full list from startIndex, so hasMoreAfter gets the
real position of the next upload.

backend/mp-lister.go

@@ -113,7 +113,8 @@ func (l *MultipartUploadLister) Run() (*ListMultipartUploadsPage, error) {
 		return emitted == l.MaxUploads
 	}
 
-	for i, up := range l.Uploads[startIndex:] {
+	for i := startIndex; i < len(l.Uploads); i++ {
+		up := l.Uploads[i]
 		if l.Delimiter != "" {
 			// delimiter check
 			suffix := strings.TrimPrefix(up.Key, l.Prefix)