tests: write commit message using file I/O

Python 2.7 will print() \x94\x5c\x0a whereas Python 3 will
print() \xc2\x94\x5c\x0a. Why, I'm not sure. It probably has to
do with print() being Unicode aware on Python 3 and Python
attempting some kind of encoding before emitting the output.

This difference results in a different bytes making it to the
commit message and the JSON output varying. We work around
this by writing bytes to a commit message file.

Differential Revision: https://phab.mercurial-scm.org/D5741

tests/test-hgweb-json.t

@@ -2196,7 +2196,8 b" Error page shouldn't crash"
 Commit message with Japanese Kanji 'Noh', which ends with '\x5c'
 
   $ echo foo >> da/foo
-  $ HGENCODING=cp932 hg ci -m `"$PYTHON" -c 'print("\x94\x5c")'`
+  >>> open('msg', 'wb').write(b'\x94\x5c\x0a') and None
+  $ HGENCODING=cp932 hg ci -l msg
 
 Commit message with null character