transaction: remove the `branch` backup for transaction

We can now back it up at the end of the transaction as we do for the rest of
the dirstate.

mercurial/dirstate.py

@@ -1765,16 +1765,12 b' class dirstate:'
 
         This is only used to do `hg rollback` related backup in the transaction
         """
-        if not self._opener.exists(self._filename):
-            # no data every written to disk yet
-            return ()
-        elif self._use_dirstate_v2:
-            return (
-                self._filename,
-                self._map.docket.data_filename(),
-            )
-        else:
-            return (self._filename,)
+        files = [b'branch']
+        if self._opener.exists(self._filename):
+            files.append(self._filename)
+            if self._use_dirstate_v2:
+                files.append(self._map.docket.data_filename())
+        return tuple(files)
 
     def verify(self, m1, m2, p1, narrow_matcher=None):
         """

mercurial/localrepo.py

@@ -2673,13 +2673,9 b' class localrepository:'
         # strip" to pick a working copy destination on `hg rollback`
         if self.currentwlock() is not None:
             ds = self.dirstate
-            if ds.branch() == b'default':
+            if not self.vfs.exists(b'branch'):
                 # force a file to be written if None exist
                 ds.setbranch(b'default', None)
-            # we cannot simply add "branch" to `all_file_names` because branch
-            # is written outside of the transaction control. So we need to
-            # backup early.
-            tr.addbackup(b"branch", hardlink=True, location=b'plain')
 
             def backup_dirstate(tr):
                 for f in ds.all_file_names():