test-check-help: fix to work on Windows

The initial problem was `hg files` prints paths with '\', which gets removed
when piped (scanhelptopics.py failed to open 'hgext__init__.py').  Then, xargs
was invoking `hg help` with 'backout\r (esc)', which setting binary mode
prevents.

tests/test-check-help.t

@@ -1,25 +1,28 b''
 #require test-repo
 
   $ . "$TESTDIR/helpers-testrepo.sh"
 
   $ cat <<'EOF' > scanhelptopics.py
   > from __future__ import absolute_import, print_function
   > import re
   > import sys
+  > if sys.platform == "win32":
+  >     import os, msvcrt
+  >     msvcrt.setmode(sys.stdout.fileno(), os.O_BINARY)
   > topics = set()
   > topicre = re.compile(r':hg:`help ([a-z0-9\-.]+)`')
   > for fname in sys.argv:
   >     with open(fname) as f:
   >         topics.update(m.group(1) for m in topicre.finditer(f.read()))
   > for s in sorted(topics):
   >     print(s)
   > EOF
 
   $ cd "$TESTDIR"/..
 
 Check if ":hg:`help TOPIC`" is valid:
 (use "xargs -n1 -t" to see which help commands are executed)
 
-  $ hg files 'glob:{hgext,mercurial}/**/*.py' \
+  $ hg files 'glob:{hgext,mercurial}/**/*.py' | sed 's|\\|/|g' \
   > | xargs python "$TESTTMP/scanhelptopics.py" \
   > | xargs -n1 hg help > /dev/null