diff --git a/mercurial/patch.py b/mercurial/patch.py
--- a/mercurial/patch.py
+++ b/mercurial/patch.py
@@ -2558,7 +2558,7 @@ def diff(
                 fctx2 is not None
             ), b'fctx2 unexpectly None in diff hunks filtering'
             hunks = hunksfilterfn(fctx2, hunks)
-        text = b''.join(sum((list(hlines) for hrange, hlines in hunks), []))
+        text = b''.join(b''.join(hlines) for hrange, hlines in hunks)
         if hdr and (text or len(hdr) > 1):
             yield b'\n'.join(hdr) + b'\n'
         if text: