Fix indentation of xstring literal (#1038)

Fixes indent calculation of this input
```
if false
p `ls`
end
```

lib/irb/nesting_parser.rb

@@ -159,7 +159,7 @@ def scan_opens(tokens)
             when :on_heredoc_end
               opens.pop
             when :on_backtick
-              opens << [t, nil] if t.state.allbits?(Ripper::EXPR_BEG)
+              opens << [t, nil] unless t.state == Ripper::EXPR_ARG
             when :on_tstring_beg, :on_words_beg, :on_qwords_beg, :on_symbols_beg, :on_qsymbols_beg, :on_regexp_beg
               opens << [t, nil]
             when :on_tstring_end, :on_regexp_end, :on_label_end

test/irb/test_nesting_parser.rb

@@ -59,6 +59,7 @@ def `; end
           def f() = 1
           %(); %w[]; %q(); %r{}; %i[]
           "#{1}"; ''; /#{1}/; `#{1}`
+          p(``); p ``; p x: ``; p 1, ``;
           :sym; :"sym"; :+; :`; :if
           [1, 2, 3]
           { x: 1, y: 2 }