hgweb: fix non-existent source or destination for diff

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hgweb: fix non-existent source or destination for diff

manifest hash: c076bc00ba04911392085f3d0697106f4d24b808
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mercurial/hgweb.py

@@ -203,12 +203,12 b' class hgweb:'
             tn = r.file(f).read(mmap2[f])
             yield diffblock(mdiff.unidiff(to, date1, tn, date2, f), f, tn)
         for f in a:
-            to = ""
+            to = None
             tn = r.file(f).read(mmap2[f])
             yield diffblock(mdiff.unidiff(to, date1, tn, date2, f), f, tn)
         for f in d:
             to = r.file(f).read(mmap1[f])
-            tn = ""
+            tn = None
             yield diffblock(mdiff.unidiff(to, date1, tn, date2, f), f, tn)
 
     def header(self):