Fix compiling on Go 1.15

Fixes https://github.com/github/hub/issues/2616

ui/format.go

 package ui
 
 import (
+	"fmt"
 	"regexp"
 	"strconv"
 	"strings"
@@ -114,7 +115,7 @@ func (f *expander) expandSpecialChar(firstChar byte, format string) (expand stri
 	case 'x':
 		if len(format) >= 2 {
 			if v, err := strconv.ParseInt(format[:2], 16, 32); err == nil {
-				return string(v), format[2:], true
+				return fmt.Sprintf("%c", v), format[2:], true
 			}
 		}
 	case '+':