subprocess: Don't add an extra whitespace at end of Windows command line

The windows code adds an unnecessary extra space to the command line.
This can cause subtle issues, so avoid it.

Github-Pull: arun11299/cpp-subprocess#119
Rebased-From: 777cfa77d1f84bb08b3e445d5f7fc6c87282223b

Author: laanwj

src/util/subprocess.h

@@ -1124,11 +1124,16 @@ inline void Popen::execute_process() noexcept(false)
   std::wstring_convert<std::codecvt_utf8_utf16<wchar_t>> converter;
   std::wstring argument;
   std::wstring command_line;
+  bool first_arg = true;
 
   for (auto arg : this->vargs_) {
+    if (!first_arg) {
+      command_line += L" ";
+    } else {
+      first_arg = false;
+    }
     argument = converter.from_bytes(arg);
     util::quote_argument(argument, command_line, false);
-    command_line += L" ";
   }
 
   // CreateProcessW can modify szCmdLine so we allocate needed memory