bookmarks: use context managers for lock and transaction in update()

Differential Revision: https://phab.mercurial-scm.org/D1839

mercurial/bookmarks.py

@@ -392,14 +392,8 b' def update(repo, parents, node):'
         bmchanges.append((bm, None))
 
     if bmchanges:
-        lock = tr = None
-        try:
-            lock = repo.lock()
-            tr = repo.transaction('bookmark')
+        with repo.lock(), repo.transaction('bookmark') as tr:
             marks.applychanges(repo, tr, bmchanges)
-            tr.close()
-        finally:
-            lockmod.release(tr, lock)
     return bool(bmchanges)
 
 def listbinbookmarks(repo):