diff: re-establish linear runtime performance

The previous method with sum() and list() creates a new list object
for every hunk. Then sum() is used to flatten out this sequence of
lists.  The sum() function is not "lazy", but creates a new list object
for every "+" operation and so this code had quadratic runtime behaviour.

mercurial/patch.py

@@ -2558,7 +2558,7 b' 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: