Enable operating on patches with unicode chars in subject lines

Currently, when run on a repository with commits containing
unicode characters in their subject lines, an exception is
generated when these patch descriptions are written to the
todo file. This patch fixes that issue by writing the todo
file using UTF-8 encoding

Change-Id: I7ed61d5432cde842b09ec106011e584dd0d5a3d0

functional-tests/040-test_simple_rebase.sh

@@ -40,7 +40,8 @@ index 170ec46..251e1dd 100644
      keywords="git hpcloud workflow",
 EOP
 
-  git commit -a -m "Add maintainer info" --quiet || return 1
+  # use smart quotes to test unicode characters in commit messages
+  git commit -a -m "Add “Davide Guerri” as maintainer" --quiet || return 1
   git push -u origin master --quiet >/dev/null || return 1
 
   log DEBUG "Cherry picking upstream commits"

git_upstream/lib/rebaseeditor.py

@@ -20,6 +20,7 @@ from git_upstream.log import LogDedentMixin
 
 from subprocess import call
 import os
+import codecs
 
 REBASE_EDITOR_SCRIPT = "rebase-editor"
 
@@ -78,7 +79,7 @@ class RebaseEditor(GitMixin, LogDedentMixin):
                 break
 
         root = None
-        with open(todo_file, "w") as todo:
+        with codecs.open(todo_file, "w", "utf-8") as todo:
             for commit in commits:
                 if not root:
                     root = commit.parents[0].hexsha