py3: make tests/test-impexp-branch.t compatible with Python 3

This patch adds some missig b'' prefixes and add suppress the output of write()
calls.

Differential Revision: https://phab.mercurial-scm.org/D3789

tests/test-impexp-branch.t

@@ -1,83 +1,83 b''
   $ echo '[extensions]' >> $HGRCPATH
   $ echo 'strip =' >> $HGRCPATH
 
   $ cat >findbranch.py <<EOF
   > from __future__ import absolute_import
   > import re
   > import sys
   > 
   > head_re = re.compile('^#(?:(?:\\s+([A-Za-z][A-Za-z0-9_]*)(?:\\s.*)?)|(?:\\s*))$')
   > 
   > for line in sys.stdin:
   >     hmatch = head_re.match(line)
   >     if not hmatch:
   >         sys.exit(1)
   >     if hmatch.group(1) == 'Branch':
   >         sys.exit(0)
   > sys.exit(1)
   > EOF
 
   $ hg init a
   $ cd a
   $ echo "Rev 1" >rev
   $ hg add rev
   $ hg commit -m "No branch."
   $ hg branch abranch
   marked working directory as branch abranch
   (branches are permanent and global, did you want a bookmark?)
   $ echo "Rev  2" >rev
   $ hg commit -m "With branch."
 
   $ hg export 0 > ../r0.patch
   $ hg export 1 > ../r1.patch
   $ cd ..
 
   $ if $PYTHON findbranch.py < r0.patch; then
   >     echo "Export of default branch revision has Branch header" 1>&2
   >     exit 1
   > fi
 
   $ if $PYTHON findbranch.py < r1.patch; then
   >     :  # Do nothing
   > else
   >     echo "Export of branch revision is missing Branch header" 1>&2
   >     exit 1
   > fi
 
 Make sure import still works with branch information in patches.
 
   $ hg init b
   $ cd b
   $ hg import ../r0.patch
   applying ../r0.patch
   $ hg import ../r1.patch
   applying ../r1.patch
   $ cd ..
 
   $ hg init c
   $ cd c
   $ hg import --exact --no-commit ../r0.patch
   applying ../r0.patch
   warning: can't check exact import with --no-commit
   $ hg st
   A rev
   $ hg revert -a
   forgetting rev
   $ rm rev
   $ hg import --exact ../r0.patch
   applying ../r0.patch
   $ hg import --exact ../r1.patch
   applying ../r1.patch
 
 Test --exact and patch header separators (issue3356)
 
   $ hg strip --no-backup .
   1 files updated, 0 files merged, 0 files removed, 0 files unresolved
   >>> import re
   >>> p = open('../r1.patch', 'rb').read()
-  >>> p = re.sub(r'Parent\s+', 'Parent ', p)
-  >>> open('../r1-ws.patch', 'wb').write(p)
+  >>> p = re.sub(br'Parent\s+', b'Parent ', p)
+  >>> open('../r1-ws.patch', 'wb').write(p) and None
   $ hg import --exact ../r1-ws.patch
   applying ../r1-ws.patch
 
   $ cd ..