Make the pg_upgrade log files contain actual commands

Now the log file not only contains the output from commands executed by
system(), but also what command it was in the first place.  This
arrangement makes debugging a lot simpler.

contrib/pg_upgrade/exec.c

@@ -29,7 +29,8 @@ static int	win32_check_directory_write_permissions(void);
  *
  *	Formats a command from the given argument list and executes that
  *	command.  If the command executes, exec_prog() returns 1 otherwise
- *	exec_prog() logs an error message and returns 0.
+ *	exec_prog() logs an error message and returns 0.  Either way, the command
+ *	line to be executed is saved to the specified log file.
  *
  *	If throw_error is TRUE, this function will throw a PG_FATAL error
  *	instead of returning should an error occur.
@@ -40,8 +41,10 @@ exec_prog(bool throw_error, bool is_priv,
 {
 	va_list		args;
 	int			result;
+	int			retval;
 	char		cmd[MAXPGPATH];
 	mode_t		old_umask = 0;
+	FILE	   *log = fopen(log_file, "a+");
 
 	if (is_priv)
 		old_umask = umask(S_IRWXG | S_IRWXO);
@@ -51,6 +54,8 @@ exec_prog(bool throw_error, bool is_priv,
 	va_end(args);
 
 	pg_log(PG_VERBOSE, "%s\n", cmd);
+	fprintf(log, "command: %s\n", cmd);
+	fflush(log);
 
 	result = system(cmd);
 
@@ -66,10 +71,15 @@ exec_prog(bool throw_error, bool is_priv,
 			   "Consult the last few lines of \"%s\" for\n"
 			   "the probable cause of the failure.\n",
 			   log_file);
-		return 1;
+		retval = 1;
 	}
+	else
+		retval = 0;
 
-	return 0;
+	fprintf(log, "\n\n");
+	fclose(log);
+
+	return retval;
 }