subprocess: Fix string_arg when used with rref

When passing in a rvalue reference, compiler
considers it ambiguous between std::string and
std::string&&. Making one of them take a lvalue
reference makes compilers correctly pick the right
one depending on whether the passed in value binds
to a rvalue or lvalue reference.

Github-Pull: arun11299/cpp-subprocess#110
Rebased-From: 2d8a8eebb03e509840e2c3c755d1abf32d930f33

Author: lunacd

src/util/subprocess.h

@@ -527,7 +527,7 @@ struct string_arg
 {
   string_arg(const char* arg): arg_value(arg) {}
   string_arg(std::string&& arg): arg_value(std::move(arg)) {}
-  string_arg(std::string arg): arg_value(std::move(arg)) {}
+  string_arg(const std::string& arg): arg_value(arg) {}
   std::string arg_value;
 };