transaction: make undoname a private attribute

This attribute tracks the file pattern to use for undo files.
It is an implementation detail of the current transaction semantics
and doesn't need to be part of the future transaction interface. So
mark it as private.

Differential Revision: https://phab.mercurial-scm.org/D4623

mercurial/transaction.py

@@ -132,7 +132,7 b' class transaction(util.transactional):'
         self.entries = []
         self.map = {}
         self.journal = journalname
-        self.undoname = undoname
+        self._undoname = undoname
         self._queue = []
         # A callback to validate transaction content before closing it.
         # should raise exception is anything is wrong.
@@ -532,9 +532,10 b' class transaction(util.transactional):'
 
     def _writeundo(self):
         """write transaction data for possible future undo call"""
-        if self.undoname is None:
+        if self._undoname is None:
             return
-        undobackupfile = self.opener.open("%s.backupfiles" % self.undoname, 'w')
+        undobackupfile = self.opener.open("%s.backupfiles" % self._undoname,
+                                          'w')
         undobackupfile.write('%d\n' % version)
         for l, f, b, c in self._backupentries:
             if not f:  # temporary file
@@ -549,7 +550,7 b' class transaction(util.transactional):'
                 vfs = self._vfsmap[l]
                 base, name = vfs.split(b)
                 assert name.startswith(self.journal), name
-                uname = name.replace(self.journal, self.undoname, 1)
+                uname = name.replace(self.journal, self._undoname, 1)
                 u = vfs.reljoin(base, uname)
                 util.copyfile(vfs.join(b), vfs.join(u), hardlink=True)
             undobackupfile.write("%s\0%s\0%s\0%d\n" % (l, f, u, c))