Fix dependent PR changes link. (#7136)

Use A..HEAD, where A is the head commit of the most recent dependency
PR. This should list all commits that are in the current PR that are not
part of that dependency commit. Produce the "warning" message if that
diff will include any commits that are in any other dependency PR.

Assisted-by: Gemini via Antigravity

github_tools/check_dependent_pr.py

@@ -47,6 +47,7 @@ _QUERY_OPEN_PRS = """
     pullRequests(states: OPEN, first: 100%(cursor)s) {
       nodes {
         number
+        headRefOid
         commits(first: 100) {
           nodes {
             commit {
@@ -282,7 +283,8 @@ def _set_commit_status(
 def _process_pr(
     client: github_helpers.Client,
     pr_number: int,
-    pr_to_commits: dict[int, list[str]],
+    pr_to_commits: dict[int, set[str]],
+    pr_to_head: dict[int, str],
     open_pr_numbers: set[int],
     label_id: str,
     token: str,
@@ -306,14 +308,13 @@ def _process_pr(
 
     open_deps: list[int] = []
 
+    current_oids = [c["commit"]["oid"] for c in commits]
+
     if len(commits) <= 1:
         _print_err(
             f"PR #{pr_number} has 1 or fewer commits, skipping overlap check."
         )
-        current_oids = [c["commit"]["oid"] for c in commits]
     else:
-        current_oids = [c["commit"]["oid"] for c in commits]
-
         # Dependency Logic: Overlap and Sequence
         #
         # We consider PR B dependent on PR A if:
@@ -327,10 +328,9 @@ def _process_pr(
         #   strict subset inclusion.
         # - Avoids circular dependencies via the sequence check.
         current_oids_set = set(current_oids)
-        for other_pr_num, other_oids in pr_to_commits.items():
+        for other_pr_num, other_oids_set in pr_to_commits.items():
             if other_pr_num >= pr_number:
                 continue
-            other_oids_set = set(other_oids)
             if not (other_oids_set & current_oids_set):
                 continue
             if not (current_oids_set - other_oids_set):
@@ -410,12 +410,16 @@ def _process_pr(
                 first_independent_commit_oid = oid
                 break
 
-        any_later_dependent_oids = False
-        if first_independent_commit_oid:
-            idx = current_oids.index(first_independent_commit_oid)
-            any_later_dependent_oids = any(
-                oid in dependent_oids for oid in current_oids[idx + 1 :]
-            )
+        last_dep_pr_num = max(open_deps)
+        last_dep_oids = pr_to_commits[last_dep_pr_num]
+        last_dep_head_oid = pr_to_head[last_dep_pr_num]
+
+        # Detect non-linear history: any commit in the current PR that is in
+        # *some* dependency but *not* in the last dependency.
+        any_later_dependent_oids = any(
+            oid in dependent_oids and oid not in last_dep_oids
+            for oid in current_oids
+        )
 
     if (
         open_deps == parsed_open_deps
@@ -439,10 +443,10 @@ def _process_pr(
 
     if open_deps:
         pr_list_str = ", ".join([f"#{num}" for num in open_deps])
-        if first_independent_commit_oid:
+        if last_dep_head_oid:
             changes_url = (
                 "https://github.com/carbon-language/carbon-lang/pull/"
-                f"{pr_number}/changes/{first_independent_commit_oid}..HEAD"
+                f"{pr_number}/changes/{last_dep_head_oid}..HEAD"
             )
             comment_body += (
                 f"Depends on {pr_list_str}, start review with "
@@ -551,7 +555,8 @@ def main() -> None:
     client = github_helpers.Client(parsed_args)
 
     _print_err("Loading open PRs ...", end="", flush=True)
-    pr_to_commits: dict[int, list[str]] = {}
+    pr_to_commits: dict[int, set[str]] = {}
+    pr_to_head: dict[int, str] = {}
     open_pr_numbers: set[int] = set()
     for node in client.execute_and_paginate(
         _QUERY_OPEN_PRS, ("repository", "pullRequests")
@@ -559,9 +564,10 @@ def main() -> None:
         _print_err(".", end="", flush=True)
         other_pr_num = node["number"]
         open_pr_numbers.add(other_pr_num)
-        pr_to_commits[other_pr_num] = [
+        pr_to_head[other_pr_num] = node["headRefOid"]
+        pr_to_commits[other_pr_num] = {
             c["commit"]["oid"] for c in node["commits"]["nodes"]
-        ]
+        }
     _print_err()
 
     label_res = client.execute(_QUERY_LABEL)
@@ -576,6 +582,7 @@ def main() -> None:
             client,
             parsed_args.pr_number,
             pr_to_commits,
+            pr_to_head,
             open_pr_numbers,
             label_id,
             parsed_args.access_token,
@@ -590,6 +597,7 @@ def main() -> None:
                 client,
                 node["number"],
                 pr_to_commits,
+                pr_to_head,
                 open_pr_numbers,
                 label_id,
                 parsed_args.access_token,