cffi: call bytes() instead of str() on CFFI buffer instances stable

cffi: call bytes() instead of str() on CFFI buffer instances

--- a/mercurial/cffi/bdiff.py	Mon Aug 05 21:08:36 2024 +0200
+++ b/mercurial/cffi/bdiff.py	Mon Aug 05 21:21:32 2024 +0200
@@ -76,7 +76,7 @@
                         lgt,
                     )
                 )
-                rl.append(str(ffi.buffer((b[0] + lb).l, lgt)))
+                rl.append(bytes(ffi.buffer((b[0] + lb).l, lgt)))
             la = h.a2
             lb = h.b2
             h = h.next