Enhanced ranking algorithm

Based on the patch by Matt Westcott (@mjwestcott).
But with a more conservative approach:
- Does not use linearly increasing penalties; It is agreed upon that we
  should prefer matching characters at the beginnings of the words, but
  it's not always clear that the relevance is inversely proportional to
  the distance from the beginning.
- The approach here is more conservative in that the bonus is never
  large enough to override the matchlen, so it can be thought of as the
  first implicit tiebreak criterion.
- One may argue the change breaks the contract of --tiebreak, but the
  judgement depends on the definition of "tie".

test/test_go.rb

@@ -207,13 +207,13 @@ class TestGoFZF < TestBase
     tmux.send_keys '99', 'C-a', '1', 'C-f', '3', 'C-b', 'C-h', 'C-u', 'C-e', 'C-y', 'C-k', 'Tab', 'BTab'
     tmux.until { |lines| lines[-2] == '  856/100000' }
     lines = tmux.capture
-    assert_equal '> 1391',       lines[-4]
+    assert_equal '> 3910',       lines[-4]
     assert_equal '  391',        lines[-3]
     assert_equal '  856/100000', lines[-2]
     assert_equal '> 391',        lines[-1]
 
     tmux.send_keys :Enter
-    assert_equal '1391', readonce.chomp
+    assert_equal '3910', readonce.chomp
   end
 
   def test_fzf_default_command
@@ -357,7 +357,7 @@ class TestGoFZF < TestBase
       tmux.send_keys :BTab, :BTab, :BTab
       tmux.until { |lines| lines[-2].include?('(3)') }
       tmux.send_keys :Enter
-      assert_equal ['5', '5', '15', '25'], readonce.split($/)
+      assert_equal ['5', '5', '50', '51'], readonce.split($/)
     end
   end
 
@@ -378,7 +378,7 @@ class TestGoFZF < TestBase
     tmux.send_keys :Enter
     tmux.until { |lines| lines[-1] == '>' }
     tmux.send_keys 'C-K', :Enter
-    assert_equal ['1919'], readonce.split($/)
+    assert_equal ['9090'], readonce.split($/)
   end
 
   def test_tac
@@ -711,10 +711,10 @@ class TestGoFZF < TestBase
 
     # len(1 ~ 2)
     output = [
-      "apple  ui     bottle 2",
       "app     ic    bottle 4",
-      "apple juice   bottle 1",
       "app     ice   bottle 3",
+      "apple  ui     bottle 2",
+      "apple juice   bottle 1",
     ]
     assert_equal output, `#{FZF} -fai -n1..2 < #{tempname}`.split($/)
 
@@ -729,9 +729,9 @@ class TestGoFZF < TestBase
 
     # len(2)
     output = [
-      "apple  ui     bottle 2",
       "app     ic    bottle 4",
       "app     ice   bottle 3",
+      "apple  ui     bottle 2",
       "apple juice   bottle 1",
     ]
     assert_equal output, `#{FZF} -fi -n2 < #{tempname}`.split($/)
@@ -943,12 +943,12 @@ class TestGoFZF < TestBase
         lines[-2].include?('/90') &&
         lines[-3]  == '  1' &&
         lines[-4]  == '  2' &&
-        lines[-13] == '> 15'
+        lines[-13] == '> 50'
       end
       tmux.send_keys :Down
     end
     tmux.send_keys :Enter
-    assert_equal '15', readonce.chomp
+    assert_equal '50', readonce.chomp
   end
 
   def test_header_lines_reverse
@@ -958,12 +958,12 @@ class TestGoFZF < TestBase
         lines[1].include?('/90') &&
         lines[2]  == '  1' &&
         lines[3]  == '  2' &&
-        lines[12] == '> 15'
+        lines[12] == '> 50'
       end
       tmux.send_keys :Up
     end
     tmux.send_keys :Enter
-    assert_equal '15', readonce.chomp
+    assert_equal '50', readonce.chomp
   end
 
   def test_header_lines_overflow