RFR: JDK-8226269: Race in SetupProcessMarkdown

[Erik Joelsson]

In SetupProcessMarkdown, if a POST_PROCESS command is given, there is a 
potential race. This is caused by the call to ExecuteWithLog in the 
recipe for the post process command where the command redirects stdout 
to a file. ExecuteWithLog documents that if such a construct is used, 
then the command must be run in a sub shell (by putting it between 
parentheses) (see JDK-8158629).

This patch adds parentheses around the affect command. I have verified 
that the exploded image no longer gets any empty man pages. This fix is 
intended for JDK 13.

Bug: https://bugs.openjdk.java.net/browse/JDK-8226269

Webrev: http://cr.openjdk.java.net/~erikj/8226269/webrev.01/

/Erik