util: enhance `nullcontextmanager` to be able to return __enter__ result
This makes its functionality and signature equivalent to Python 3.7’s
contextlib.nullcontext().
--- a/mercurial/util.py Fri Jul 17 03:12:29 2020 +0200
+++ b/mercurial/util.py Thu Jul 16 23:49:04 2020 +0200
@@ -1381,8 +1381,8 @@
@contextlib.contextmanager
-def nullcontextmanager():
- yield
+def nullcontextmanager(enter_result=None):
+ yield enter_result
class _lrucachenode(object):