[common] Add a way to set the process startup directory (#63)

This will be needed later for switching to sftp, since calling lcd in
sftp is tricky to get right (e.g. may or may not require /cygwin/c on
Windows, depending on whether sftp is native or not).

common/process.h

@@ -53,6 +53,10 @@ struct ProcessStartInfo {
   // Command line, UTF-8 encoded.
   std::string command;
 
+  // Full path to the process startup working directory.
+  // If empty, uses parent process working dir.
+  std::string startup_dir;
+
   // If set, the process stdin is redirected to a pipe.
   // It not set, the input is connected to the stdin of the calling process.
   bool redirect_stdin = false;