This test has been failing on Windows since 0af56d3ee24c
introduced the hg files invocation. Specifically, Windows seems
to be choking on special characters in the fileset pattern. I
believe at least \n and > were causing issues.
I attempted various incantations to make the Windows command line
parser accept the fileset but couldn't get anything working.
I declared bankruptcy and just reimplemented the grepping code
After this change, the test now passes on Windows again.