util.h: re-indent function string_format and use const std::string & for read_file (#19931)

old-commit-hash: 316f475544

selfdrive/common/util.h

@@ -46,7 +46,7 @@ inline std::string string_format( const std::string& format, Args ... args ) {
   return std::string(buf.get(), buf.get() + size - 1);
 }
 
-inline std::string read_file(std::string fn) {
+inline std::string read_file(const std::string &fn) {
   std::ifstream t(fn);
   std::stringstream buffer;
   buffer << t.rdbuf();