transaction: drop backupentries logic from startgroup and endgroup

The `startgroup` and `endgroup` methods are used in a very specific
context to wrap a very specific operation (revlog truncation). It does
not make sense to perform any other operations during such a "group"
(eg:file backup). There is currently no user of backupfile during a
"group" so we drop the group-specific code and restrict authorized
operations during "group".

mercurial/transaction.py

@@ -121,7 +121,7 b' class transaction(object):'
 
         This is used by strip to delay vision of strip offset. The transaction
         sees either none or all of the strip actions to be done."""
-        self._queue.append(([], []))
+        self._queue.append([])
 
     @active
     def endgroup(self):
@@ -130,31 +130,22 b' class transaction(object):'
         This is used by strip to delay vision of strip offset. The transaction
         sees either none or all of the strip actions to be done."""
         q = self._queue.pop()
-        self.entries.extend(q[0])
-        self._backupentries.extend(q[1])
+        self.entries.extend(q)
 
         offsets = []
-        backups = []
-        for f, o, _data in q[0]:
+        for f, o, _data in q:
             offsets.append((f, o))
 
-        for f, b in q[1]:
-            backups.append((f, b))
-
         d = ''.join(['%s\0%d\n' % (f, o) for f, o in offsets])
         self.file.write(d)
         self.file.flush()
 
-        d = ''.join(['%s\0%s\n' % (f, b) for f, b in backups])
-        self._backupsfile.write(d)
-        self._backupsfile.flush()
-
     @active
     def add(self, file, offset, data=None):
         if file in self.map or file in self._backupmap:
             return
         if self._queue:
-            self._queue[-1][0].append((file, offset, data))
+            self._queue[-1].append((file, offset, data))
             return
 
         self.entries.append((file, offset, data))
@@ -174,6 +165,9 b' class transaction(object):'
         * `file`: the file path, relative to .hg/store
         * `hardlink`: use a hardlink to quickly create the backup
         """
+        if self._queue:
+            msg = 'cannot use transaction.addbackup inside "group"'
+            raise RuntimeError(msg)
 
         if file in self.map or file in self._backupmap:
             return
@@ -188,10 +182,6 b' class transaction(object):'
             self.add(file, 0)
             return
 
-        if self._queue:
-            self._queue[-1][1].append((file, backupfile))
-            return
-
         self._backupentries.append((file, backupfile))
         self._backupmap[file] = len(self._backupentries) - 1
         self._backupsfile.write("%s\0%s\n" % (file, backupfile))