Bail on attempts to do an empty commit

mercurial/hg.py

@@ -298,8 +298,6 b' class localrepository:'
                            self.join("undo"))
 
     def commit(self, parent, update = None, text = ""):
-        tr = self.transaction()
-        
         try:
             remove = [ l[:-1] for l in self.opener("to-remove") ]
             os.unlink(self.join("to-remove"))
@@ -310,6 +308,12 b' class localrepository:'
         if update == None:
             update = self.diffdir(self.root, parent)[0]
 
+        if not update:
+            self.ui.status("nothing changed\n")
+            return
+
+        tr = self.transaction()
+
         # check in files
         new = {}
         linkrev = self.changelog.count()