Update remote-tracking branches when processing logs

Since we're explicitly pulling only master with `git pull origin master`
instead of just `git pull`, remote-tracking branches are not updated
by the pull. This means that on checking out master after processing
and pushing, we see "Your branch is ahead of 'origin/master' by [n]
commits" if master had been updated before the pull. This appears to
suggest that we've just committed to master and haven't yet pushed,
if you're used to having your remote-tracking branches up-to-date.
Fetching first removes this potentially ambiguous message.
Merge pull request #124 from alphagov/git_fetch_before_processing_logs

Update remote-tracking branches when processing logs