chore: merge latest changes when asserting changes

[wardpeet]

Description

We see a lot of failing PRs happening lately because of our starters_validate step. It diffs the branch with the latest master and sees if any changes have happened on starters/ or .circle folders. Most of our publishes change the starters folder which means all newly created PRs who didn't start from master will fail on this test.

#!/bin/bash -eo pipefail
./scripts/assert-changed-files.sh "starters/*|.circleci/*"
1 file(s) matching 'starters/*|.circleci/*'; continuing.
CircleCI received exit code 0

I altered the script to git merge the latest master before we do the check to see if any real changes were made. This should reduce the number of failures.

[blainekasten]

I'm going to approve because this all looks reasonable and good. A couple nits that I think could be simplified, but it's ultimately nit. :)