Fix ANSI attributes lost when nth:regular is set

Example:
  # foo was not displayed in italic
  echo -e "\x1b[33;3mfoo \x1b[mbar" | fzf --ansi --color fg:dim,nth:regular --nth 1

src/result.go

@@ -228,10 +228,11 @@ func (result *Result) colorOffsets(matchOffsets []Offset, nthOffsets []Offset, t
 					offset: [2]int32{int32(start), int32(idx)}, color: color, match: true, url: url})
 			} else if curr.color {
 				ansi := itemColors[curr.index]
-				color := ansiToColorPair(ansi, colBase)
+				base := colBase
 				if curr.nth {
-					color = color.WithAttr(attrNth)
+					base = base.WithAttr(attrNth)
 				}
+				color := ansiToColorPair(ansi, base)
 				colors = append(colors, colorOffset{
 					offset: [2]int32{int32(start), int32(idx)},
 					color:  color,

src/result_test.go

@@ -176,7 +176,7 @@ func TestColorOffset(t *testing.T) {
 		assert(9, 35, 37, tui.NewColorPair(4, 8, tui.Bold))
 		expected := tui.Bold | attr
 		if attr == tui.AttrRegular {
-			expected = tui.AttrRegular
+			expected = tui.Bold
 		}
 		assert(10, 37, 39, tui.NewColorPair(4, 8, expected))
 		assert(11, 39, 40, tui.NewColorPair(4, 8, tui.Bold))