[ruby/irb] Fix crashing when multiple open braces per line

https://github.com/ruby/irb/issues/55

If we had put multiple open braces on a line the with no closing brace
spaces_of_nest array keeps getting '0' added to it. This means that when
we pop off of this array we are saying that we should be in position zero
for the next line. This is an issue because we don't always want to be
in position 0 after a closing brace.

Example:
```
[[[
]
]
]
```
In the above example the 'spaces_of_nest' array looks like this after
the first line is entered: [0,0,0]. We really want to be indented 4
spaces for the 1st closing brace 2 for the 2nd and 0 for the 3rd. i.e.
we want it to be: [0,2,4].

We also saw this issue with a heredoc inside of an array.

```
[<<FOO]
hello
FOO
```

https://github.com/ruby/irb/commit/80c69c8272

lib/irb/ruby-lex.rb

@@ -368,6 +368,7 @@ class RubyLex
     is_first_printable_of_line = true
     spaces_of_nest = []
     spaces_at_line_head = 0
+    open_brace_on_line = 0
     @tokens.each_with_index do |t, index|
       case t[1]
       when :on_ignored_nl, :on_nl, :on_comment
@@ -375,6 +376,7 @@ class RubyLex
         spaces_at_line_head = 0
         is_first_spaces_of_line = true
         is_first_printable_of_line = true
+        open_brace_on_line = 0
         next
       when :on_sp
         spaces_at_line_head = t[2].count(' ') if is_first_spaces_of_line
@@ -383,7 +385,8 @@ class RubyLex
       end
       case t[1]
       when :on_lbracket, :on_lbrace, :on_lparen
-        spaces_of_nest.push(spaces_at_line_head)
+        spaces_of_nest.push(spaces_at_line_head + open_brace_on_line * 2)
+        open_brace_on_line += 1
       when :on_rbracket, :on_rbrace, :on_rparen
         if is_first_printable_of_line
           corresponding_token_depth = spaces_of_nest.pop

test/irb/test_ruby_lex.rb

@@ -93,5 +93,24 @@ module TestIRB
         assert_indenting(lines, row.new_line_spaces, true)
       end
     end
+
+    def test_multiple_braces_in_a_line
+      input_with_correct_indents = [
+        Row.new(%q([[[), nil, 6),
+        Row.new(%q(    ]), 4, 4),
+        Row.new(%q(  ]), 2, 2),
+        Row.new(%q(]), 0, 0),
+        Row.new(%q([<<FOO]), nil, 0),
+        Row.new(%q(hello), nil, 0),
+        Row.new(%q(FOO), nil, 0),
+      ]
+
+      lines = []
+      input_with_correct_indents.each do |row|
+        lines << row.content
+        assert_indenting(lines, row.current_line_spaces, false)
+        assert_indenting(lines, row.new_line_spaces, true)
+      end
+    end
   end
 end