[ez][trymerge] Edit revert message for reverted ghstack PRs (#153573)

Change comment about successful revert so it also contains info about the original PR that got the comment (if it is a ghstacked PR)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/153573
Approved by: https://github.com/atalman, https://github.com/malfet

.github/scripts/trymerge.py

@@ -1938,6 +1938,7 @@ def get_ghstack_dependent_prs(
 
 def do_revert_prs(
     repo: GitRepo,
+    original_pr: GitHubPR,
     shas_and_prs: list[tuple[str, GitHubPR]],
     *,
     author_login: str,
@@ -1959,9 +1960,16 @@ def do_revert_prs(
 
     # Comment/reopen PRs
     for commit_sha, pr in shas_and_prs:
-        revert_message = (
-            f"@{pr.get_pr_creator_login()} your PR has been successfully reverted."
-        )
+        revert_message = ""
+        if pr.pr_num == original_pr.pr_num:
+            revert_message += (
+                f"@{pr.get_pr_creator_login()} your PR has been successfully reverted."
+            )
+        else:
+            revert_message += (
+                f"@{pr.get_pr_creator_login()} your PR has been reverted as part of the stack under "
+                f"#{original_pr.pr_num}.\n"
+            )
         if (
             pr.has_internal_changes()
             and not pr.has_no_connected_diff()
@@ -2013,6 +2021,7 @@ def try_revert(
 
     do_revert_prs(
         repo,
+        pr,
         shas_and_prs,
         author_login=author_login,
         extra_msg=extra_msg,