[EngSys] improve PR Diff Generation in pull request test builds

Currently it is comparing pull request source commit id against target branch. This requires git history and prevent PR pipelines from doing shallow clone.

https://github.com/Azure/azure-sdk-for-js/blob/e5aac771365063170a880a73b2971b744c63079a/eng/common/scripts/Helpers/git-helpers.ps1#L34-L35

It looks that with fetchDepth of 2, we can compare `HEAD^...HEAD` to get the changed files. https://dev.azure.com/azure-sdk/public/_build/results?buildId=5302405&view=logs&j=7fd4082b-93bd-5b39-44dc-2897910c0398&t=fecda729-0cc7-5830-9299-54206a067b4a

Start in eng/common/scripts/Helpers/git-helpers.ps1 at lines 34-35 and inspect how pull request test builds determine the source and target commits. Check the linked build log and verify the changed-file comparison works with fetchDepth 2 using HEAD^...HEAD; done means PR pipelines no longer require full git history while producing the correct diff.

Written by the indexing model from the issue text.