In Python 3, you have to be careful to distinguish between
bytes and strings when reading from subprocesses; the code
to read the commit position was failing to do that in one place.

Bug: chromium:1112471
Change-Id: Ifbfba6872f762d7e5d7e356772e93b4a7ee1dfeb
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2508746
Commit-Queue: Dirk Pranke <dpranke@google.com>
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>