Never emit an empty "id" anchor as it is not possible to link to "#" (vs...

Never emit an empty "id" anchor as it is not possible to link to "#" (vs "#foo"). We use "#top" as the fallback so it will work with the top-level container.

diffoscope/presenters/html/html.py

@@ -115,7 +115,7 @@ def output_diff_path(path):
 
 
 def output_anchor(path):
-    return escape_anchor(output_diff_path(path))
+    return escape_anchor(output_diff_path(path)) or "top"
 
 
 def convert(s, ponct=0, tag=''):

tests/data/output_regression_875281.html

@@ -4,8 +4,8 @@
   <div class="diffcontrol diffcontrol-double">⊟</div>
   <div><span class="diffsize">4.82 MB</span></div>
   <div><span class="source">b1</span> vs.</div>
-  <div ><span class="source">b2</span>
-    <a class="anchor" href="#">¶</a>
+  <div id="top"><span class="source">b2</span>
+    <a class="anchor" href="#top">¶</a>
   </div>
   </div>
   <div class="difference">