Fix printing error on git-rebase failure

R=ehmaldonado@chromium.org

Bug: 1071280
Change-Id: I8b1f00bb7c454297b10d5e047ae35c2686f69fb1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2155154
Reviewed-by: Edward Lesmes <ehmaldonado@chromium.org>
Commit-Queue: Josip Sokcevic <sokcevic@google.com>

git_rebase_update.py

@@ -197,8 +197,8 @@ def rebase_branch(branch, parent, start_hash):
         else:
           print("Here's what git-rebase (squashed) had to say:")
           print()
-          print(squash_ret.stdout)
-          print(squash_ret.stderr)
+          print(squash_ret.stdout.decode('utf-8'))
+          print(squash_ret.stderr.decode('utf-8'))
           print(textwrap.dedent("""\
           Squashing failed. You probably have a real merge conflict.