Add new tiebreak: 'chunk'

Favors the line with shorter matched chunk. A chunk is a set of
consecutive non-whitespace characters.

Unlike the default `length`, this new scheme works well with tabular input.

  # length prefers item #1, because the whole line is shorter,
  # chunk prefers item #2, because the matched chunk ("foo") is shorter
  fzf --height=6 --header-lines=2 --tiebreak=chunk --reverse --query=fo << "EOF"
  N | Field1 | Field2 | Field3
  - | ------ | ------ | ------
  1 | hello  | foobar | baz
  2 | world  | foo    | bazbaz
  EOF

If the input does not contain any spaces, `chunk` is equivalent to
`length`. But we're not going to set it as the default because it is
computationally more expensive.

Close #2285
Close #2537
- Not the exact solution to --tiebreak=length not taking --nth into account,
  but this should work. And the added benefit is that it works well even
  when --nth is not provided.
- Adding a bonus point to the last character of a word didn't turn out great.
  The order of the result suddenly changes when you type in the last
  character in the word producing a jarring effect.

test/test_go.rb

@@ -754,6 +754,20 @@ class TestGoFZF < TestBase
     assert_equal output, `#{FZF} -fh -n2 -d: < #{tempname}`.lines(chomp: true)
   end
 
+  def test_tiebreak_chunk
+    writelines(tempname, [
+                 '1 foobarbaz baz',
+                 '2 foobar baz',
+                 '3 foo barbaz'
+               ])
+
+    assert_equal [
+      '3 foo barbaz',
+      '2 foobar baz',
+      '1 foobarbaz baz'
+    ], `#{FZF} -fo --tiebreak=chunk < #{tempname}`.lines(chomp: true)
+  end
+
   def test_invalid_cache
     tmux.send_keys "(echo d; echo D; echo x) | #{fzf('-q d')}", :Enter
     tmux.until { |lines| assert_equal '  2/3', lines[-2] }