obsolete: properly increment currentlen when building pushkey payload

In the old code, the current length was always 0 leading to markers
payload never being split.

mercurial/obsolete.py

@@ -273,6 +273,7 b' def listmarkers(repo):'
             currentlen = 0
             parts.append(currentpart)
         currentpart.append(nextdata)
+        currentlen += len(nextdata)
     for idx, part in enumerate(reversed(parts)):
         data = ''.join([_pack('>B', _fmversion)] + part)
         keys['dump%i' % idx] = base85.b85encode(data)