util: enhance `nullcontextmanager` to be able to return __enter__ result

This makes its functionality and signature equivalent to Python 3.7’s
contextlib.nullcontext().

mercurial/util.py

@@ -1381,8 +1381,8 b' def acceptintervention(tr=None):'
 
 
 @contextlib.contextmanager
-def nullcontextmanager():
-    yield
+def nullcontextmanager(enter_result=None):
+    yield enter_result
 
 
 class _lrucachenode(object):