tests: use `f --hexdump` to print file content

The inline print.py in this test wasn't fully compatible with
Python 3 because it was reading from sys.stdin, which already
normalized line endings since it operates in the realm of str on
Python 3. To do this correctly, we'd need to read from
sys.stdin.buffer on Python 3. This would entail conditional code.
I felt this was too much effort. So I just replaced the custom
script with `f`, which already knows how to do the right thing.

test-mactext.t now passes on Python 3 on Windows.

Differential Revision: https://phab.mercurial-scm.org/D8336

tests/test-mactext.t

@@ -7,10 +7,6 b''
   >     data = data.replace(b'\n', b'\r')
   >     open(path, 'wb').write(data)
   > EOF
-  $ cat > print.py <<EOF
-  > import sys
-  > print(sys.stdin.read().replace('\n', '<LF>').replace('\r', '<CR>').replace('\0', '<NUL>'))
-  > EOF
   $ hg init
   $ echo '[hooks]' >> .hg/hgrc
   $ echo 'pretxncommit.cr = python:hgext.win32text.forbidcr' >> .hg/hgrc
@@ -32,7 +28,9 b''
   rollback completed
   abort: pretxncommit.cr hook failed
   [255]
-  $ hg cat f | "$PYTHON" print.py
-  hello<LF>
-  $ cat f | "$PYTHON" print.py
-  hello<CR>
+  $ hg cat f | f --hexdump
+  
+  0000: 68 65 6c 6c 6f 0a                               |hello.|
+  $ f --hexdump f
+  f:
+  0000: 68 65 6c 6c 6f 0d                               |hello.|