testparseutil: escape backslash in docstring

This is funky. This inline Python code is part of a docstring, which
means the string is interpreted first. So any backslashes need double
escaping. So even though this is already a br'', we still need to escape.

Differential Revision: https://phab.mercurial-scm.org/D5842

contrib/testparseutil.py

@@ -265,7 +265,7 b" heredoclimitpat = br'\\s*<<\\s*(?P<lquote>"
 class fileheredocmatcher(embeddedmatcher):
     """Detect "cat > FILE << LIMIT" style embedded code
 
-    >>> matcher = fileheredocmatcher(b'heredoc .py file', br'[^<]+\.py')
+    >>> matcher = fileheredocmatcher(b'heredoc .py file', br'[^<]+\\.py')
     >>> b2s(matcher.startsat(b'  $ cat > file.py << EOF\\n'))
     ('file.py', '  > EOF\\n')
     >>> b2s(matcher.startsat(b'  $ cat   >>file.py   <<EOF\\n'))