bookmarks: Only save undo.bookmarks if bookmarks exist
Otherwise the command will abort when there is no .hg/bookmarks file.
% should fail
abort: path contains illegal component: .hg/00changelog.i
adding a/a
% should fail
abort: path 'b/b' traverses symbolic link 'b'
% should succeed
% should still fail - maybe
abort: path 'b/b' traverses symbolic link 'b'