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

This is the way.

hgext/gpg.py

@@ -352,6 +352,7 def _dosign(ui, repo, *revs, **opts):
     sigsfile.close()
 
     if b'.hgsigs' not in repo.dirstate:
+        with repo.dirstate.changing_files(repo):
         repo[None].add([b".hgsigs"])
 
     if opts[b"no_commit"]: