ci: fix scratch build change detection commit calculation

Author: dmcilvaney

.github/workflows/ado/templates/pr-package-build-stages.yml

@@ -153,25 +153,35 @@ stages:
 
           # Resolve the PR commit range. A PR-policy build checks out the MERGE
           # commit (Build.SourceVersion): parent ^1 is the target-branch tip,
-          # parent ^2 is the PR head. The diff ^1..^2 is exactly the PR's
-          # changes relative to the target branch. We read the range here and
-          # set pipeline variables so the wiring stays visible in the YAML.
+          # parent ^2 is the PR head. The PR's true change set is the diff
+          # between their MERGE BASE (the fork point) and ^2 -- i.e.
+          # `git diff merge-base(^1,^2)..^2`, the same three-dot set GitHub's
+          # "Files changed" shows. We deliberately compare against the merge
+          # base, NOT the target tip: comparing against the tip additionally
+          # flags every component that landed on the target branch after the PR
+          # forked (the PR head still carries their pre-update state), which can
+          # balloon a 1-component PR into dozens. We read the range here and set
+          # pipeline variables so the wiring stays visible in the YAML.
           - script: |
               set -euo pipefail
               if ! git rev-parse --verify -q "HEAD^2" >/dev/null; then
                 echo "##[error]HEAD is not a merge commit -- this pipeline must run as a PR build (Build.Reason=PullRequest)."
                 exit 1
               fi
-              target_commit="$(git rev-parse HEAD^1)"
               source_commit="$(git rev-parse HEAD^2)"
+              # Baseline = the MERGE BASE (fork point) of the target-branch tip
+              # (HEAD^1) and the PR head (HEAD^2), NOT the target tip itself.
+              # See the block comment above for why; this is passed as the
+              # `--target-commit` (from) baseline to the change-set helper.
+              target_commit="$(git merge-base HEAD^1 HEAD^2)"
               # PR-supplied data is untrusted: validate both SHAs before use.
               for sha in "$target_commit" "$source_commit"; do
                 if [[ ! "$sha" =~ ^[0-9a-f]{40}$ ]]; then
                   echo "##[error]invalid commit SHA: $sha"
                   exit 1
                 fi
               done
-              echo "Resolved range: target=$target_commit source=$source_commit"
+              echo "Resolved range: base(merge-base)=$target_commit source=$source_commit"
               echo "##vso[task.setvariable variable=sourceCommit;isreadonly=true]$source_commit"
               echo "##vso[task.setvariable variable=targetCommit;isreadonly=true]$target_commit"
             displayName: "Determine PR commit range"