tests: perform grep manually in test-doctest.py 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 in Python. After this change, the test now passes on Windows again. Differential Revision: https://phab.mercurial-scm.org/D8343