Race in SetupProcessMarkdown

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 in parenthesis) (see JDK-8158629).

I don't think this affects anything important, but there is potential. If building man pages (as in the closed Oracle build) then some man pages in the exploded image will be empty files. These files are never used so shouldn't matter. There is also the potential for producing a corrupt docs bundle, though I suspect this is very rare. Because of the latter, I think this is important enough to be fixed in 13.