also test album sorting with negative numbers

src/sigal/gallery.py

@@ -515,7 +515,7 @@ class Album:
                         continue
                 return ""
 
-            key = natsort_keygen(key=sort_key, alg=ns.LOCALE)
+            key = natsort_keygen(key=sort_key, alg=ns.SIGNED|ns.LOCALE)
             self.subdirs.sort(key=key, reverse=reverse)
 
         signals.albums_sorted.send(self)

tests/sample/pictures/dir1/test2/index.md

@@ -1 +1,2 @@
-Order: 01
+Order: -10
+PartialOrder: 01

tests/test_gallery.py

@@ -234,8 +234,7 @@ def test_albums_sort(settings):
     a.sort_subdirs("title")
     assert [im.title for im in a.albums] == list(reversed(titles))
 
-    orders = ["01", "02", "03"]
-    orders.sort()
+    orders = ["-10", "02", "03"]
     settings["albums_sort_reverse"] = False
     a = Album("dir1", settings, album["subdirs"], album["medias"], gal)
     a.sort_subdirs("meta.order")