[Swagger BreakingChange] Unreliable calculation of "git diff" for PR

[mikeharder]

PR: #33021

Incorrect calculation of "git diff" on first commit to PR: https://dev.azure.com/azure-sdk/internal/_build/results?buildId=4620973&view=logs&j=98f87804-8e1f-5655-af02-e80aefa7aa97&t=65457206-7ab6-5966-2a2f-9ae3efa14a03&l=24

Correct calculation of "git diff'" after pushing second commit to PR: https://dev.azure.com/azure-sdk/internal/_build/results?buildId=4621391&view=logs&j=98f87804-8e1f-5655-af02-e80aefa7aa97&t=65457206-7ab6-5966-2a2f-9ae3efa14a03&l=24

Commit to main that triggered race condition bug: 1175ddb

Root cause, the check code diffs the PR branch against the base branch, not the base SHA. Meaning, if a commit is merged to the base branch after the check has started, the diff will be incorrect, since it includes commits in main but not yet in the merge SHA.

The code should be changed to use the canonical version for this logic, which is currently here:

azure-rest-api-specs/.github/workflows/src/changed-files.js

Lines 12 to 21 in 6043a6a

	export async function getChangedFiles(
	core,
	baseCommitish = "HEAD^",
	headCommitish = "HEAD",
	diffFilter = "d",
	) {
	const result = await execRoot(
	`git -c core.quotepath=off diff --name-only --diff-filter=${diffFilter} ${baseCommitish} ${headCommitish}`,
	core,
	);