fix(labels): compare release shape to merge base

actions/labels-reconcile/labels-reconcile.sh

@@ -270,7 +270,8 @@ set_required_bots() { # the PR author is recused by construction
 # The state machine. Pure functions over these globals, set per PR:
 #   DRAFT        true|false
 #   HEAD_SHA     the PR's current head commit
-#   BASE_SHA     the PR's base branch head (the release-shape guard's ref)
+#   BASE_SHA     the PR's base branch head
+#   MERGE_BASE_SHA  the PR's merge base (the release-shape guard's ref)
 #   REQUESTED    newline-separated logins with a review currently requested
 #   REVIEWS_JSON JSON array of submitted, gradeable reviews
 #   MERGEABLE    MERGEABLE | CONFLICTING | UNKNOWN  (GitHub's own verdict)
@@ -918,7 +919,8 @@ reconcile_pr() { # $1 = PR number; relies on the globals set from its fetch
   # Drafts are exempt (the build phase is the builder's); the version
   # reads cost two API calls and only on PRs missing the label.
   if [ "$DRAFT" != true ] && ! has_label release; then
-    release_shape_warning "$n" "$(tree_version "$HEAD_SHA")" "$(tree_version "$BASE_SHA")"
+    release_shape_warning "$n" "$(tree_version "$HEAD_SHA")" \
+      "$(tree_version "${MERGE_BASE_SHA:-$BASE_SHA}")"
   fi
 
   # ---- merge-next: cleared, never set ----------------------------------
@@ -1031,6 +1033,7 @@ main() {
       set_required_bots "$AUTHOR"
       HEAD_SHA="$(jq -r '.head.sha' <<<"$PR_JSON")"
       BASE_SHA="$(jq -r '.base.sha' <<<"$PR_JSON")"
+      MERGE_BASE_SHA="$(jq -r '.merge_base // empty' <<<"$PR_JSON")"
       LABELS="$(jq -r '.labels[].name' <<<"$PR_JSON")"
       # This allow-list answers whether a row is a submitted, gradeable review;
       # bot_verdict separately answers what that submitted verdict says (#235).

changelog.d/275.md

@@ -0,0 +1 @@
+- Compare release-shaped pull requests with their merge base so later base-branch releases do not create phantom version-change warnings (#275).