exchange: use context manager for bundle1 unbundling

The lazy locking is not used for bundle1, so using a regular context
manager is clearer.

mercurial/exchange.py

@@ -1733,7 +1733,7 def unbundle(repo, cg, heads, source, ur
         # push can proceed
         if not util.safehasattr(cg, 'params'):
             # legacy case: bundle1 (changegroup 01)
-            lockandtr[1] = repo.lock()
+            with repo.lock():
             r = cg.apply(repo, source, url)
         else:
             r = None