dirstate: add prefix and suffix arguments to backup

dirstate: add prefix and suffix arguments to backup

This would allow the code explicitly copying dirstate to use this method instead.
Use of this method will increase encapsulation (the dirstate class will be sole
owner of its on-disk storage).

--- a/mercurial/dirstate.py	Wed Apr 20 19:55:59 2016 +0000
+++ b/mercurial/dirstate.py	Fri May 13 13:28:09 2016 -0700
@@ -1207,7 +1207,7 @@
         else:
             return self._filename
 
-    def savebackup(self, tr, suffix):
+    def savebackup(self, tr, suffix='', prefix=''):
         '''Save current dirstate into backup file with suffix'''
         filename = self._actualfilename(tr)
 
@@ -1228,17 +1228,18 @@
             # end of this transaction
             tr.registertmp(filename, location='plain')
 
-        self._opener.write(filename + suffix, self._opener.tryread(filename))
+        self._opener.write(prefix + filename + suffix,
+                           self._opener.tryread(filename))
 
-    def restorebackup(self, tr, suffix):
+    def restorebackup(self, tr, suffix='', prefix=''):
         '''Restore dirstate by backup file with suffix'''
         # this "invalidate()" prevents "wlock.release()" from writing
         # changes of dirstate out after restoring from backup file
         self.invalidate()
         filename = self._actualfilename(tr)
-        self._opener.rename(filename + suffix, filename)
+        self._opener.rename(prefix + filename + suffix, filename)
 
-    def clearbackup(self, tr, suffix):
+    def clearbackup(self, tr, suffix='', prefix=''):
         '''Clear backup file with suffix'''
         filename = self._actualfilename(tr)
-        self._opener.unlink(filename + suffix)
+        self._opener.unlink(prefix + filename + suffix)