dirstate: use `dirstate.change_files` to scope the change in `tag`

This is the way.

mercurial/tags.py

@@ -664,6 +664,7 b' def _tag('
 
     repo.invalidatecaches()
 
+    with repo.dirstate.changing_files(repo):
     if b'.hgtags' not in repo.dirstate:
         repo[None].add([b'.hgtags'])