exchange: fix locking to actually be scoped

The previous code was taking locks before entering with statements, so
exception before the with statement would not release the lock (except for
garbage collection).

mercurial/exchange.py

@@ -1694,10 +1694,10 b' def pull('
             )
 
     pullop.trmanager = transactionmanager(repo, b'pull', remote.url())
-    wlock = util.nullcontextmanager()
+    wlock = util.nullcontextmanager
     if not bookmod.bookmarksinstore(repo):
-        wlock = repo.wlock()
-    with wlock, repo.lock(), pullop.trmanager:
+        wlock = repo.wlock
+    with wlock(), repo.lock(), pullop.trmanager:
         if confirm or (
             repo.ui.configbool(b"pull", b"confirm") and not repo.ui.plain()
         ):