transaction: trivial refactoring

Differential Revision: https://phab.mercurial-scm.org/D10714

mercurial/transaction.py

@@ -720,9 +720,8 b' def rollback(opener, vfsmap, file, repor'
     entries = []
     backupentries = []
 
-    fp = opener.open(file)
+    with opener.open(file) as fp:
     lines = fp.readlines()
-    fp.close()
     for l in lines:
         try:
             f, o = l.split(b'\0')