transaction: gather backupjournal logic together in the __init__

The initialisation of file-backup related variable were a bit scattered, we
gather them together.

mercurial/transaction.py

@@ -91,19 +91,20 b' class transaction(object):'
         self.onabort = onabort
         self.entries = []
         self.map = {}
+        self.journal = journal
+        self._queue = []
+        # a dict of arguments to be passed to hooks
+        self.hookargs = {}
+        self.file = opener.open(self.journal, "w")
+
         # a list of ('path', 'backuppath') entries.
         # if 'backuppath' is empty, no file existed at backup time
         self._backupentries = []
         self._backupmap = {}
-        self.journal = journal
-        self._queue = []
-        # a dict of arguments to be passed to hooks
-        self.hookargs = {}
-
         self._backupjournal = "%s.backupfiles" % journal
-        self.file = opener.open(self.journal, "w")
         self._backupsfile = opener.open(self._backupjournal, 'w')
         self._backupsfile.write('%d\n' % version)
+
         if createmode is not None:
             opener.chmod(self.journal, createmode & 0666)
             opener.chmod(self._backupjournal, createmode & 0666)