[GHF] Add support for new style stacks (#116873)

Where base stack targets default branch, rather than base. But as
default branch is likely to advance, since PR was made, search for
mergebase before determining whether `base`..`head` are in sync with `orig` branch
Also, rather than hardcode default branch name, fetch it from `GitHubPR.default_branch()`

Test Plan: https://github.com/malfet/deleteme/pull/77

Pull Request resolved: https://github.com/pytorch/pytorch/pull/116873
Approved by: https://github.com/ezyang

.github/scripts/gitutils.py

@@ -397,13 +397,28 @@ def _shasum(value: str) -> str:
     return m.hexdigest()
 
 
-def are_ghstack_branches_in_sync(repo: GitRepo, head_ref: str) -> bool:
+def is_commit_hash(ref: str) -> bool:
+    "True if ref is hexadecimal number, else false"
+    try:
+        int(ref, 16)
+    except ValueError:
+        return False
+    return True
+
+
+def are_ghstack_branches_in_sync(
+    repo: GitRepo, head_ref: str, base_ref: Optional[str] = None
+) -> bool:
     """Checks that diff between base and head is the same as diff between orig and its parent"""
     orig_ref = re.sub(r"/head$", "/orig", head_ref)
-    base_ref = re.sub(r"/head$", "/base", head_ref)
+    if base_ref is None:
+        base_ref = re.sub(r"/head$", "/base", head_ref)
     orig_diff_sha = _shasum(repo.diff(f"{repo.remote}/{orig_ref}"))
     head_diff_sha = _shasum(
-        repo.diff(f"{repo.remote}/{base_ref}", f"{repo.remote}/{head_ref}")
+        repo.diff(
+            base_ref if is_commit_hash(base_ref) else f"{repo.remote}/{base_ref}",
+            f"{repo.remote}/{head_ref}",
+        )
     )
     return orig_diff_sha == head_diff_sha
 

.github/scripts/trymerge.py

@@ -674,10 +674,15 @@ def get_ghstack_prs(
     for stacked_pr, rev in entire_stack:
         if stacked_pr.is_closed():
             continue
-        if not are_ghstack_branches_in_sync(repo, stacked_pr.head_ref()):
+        base_ref = stacked_pr.base_ref()
+        if base_ref == pr.default_branch():
+            base_ref = repo.get_merge_base(
+                f"{repo.remote}/{base_ref}", f"{repo.remote}/{stacked_pr.head_ref()}"
+            )
+        if not are_ghstack_branches_in_sync(repo, stacked_pr.head_ref(), base_ref):
             raise RuntimeError(
                 f"PR {stacked_pr.pr_num} is out of sync with the corresponding revision {rev} on "
-                + f"branch {orig_ref} that would be merged into main.  "
+                + f"branch {stacked_pr.get_ghstack_orig_ref()} that would be merged into {stacked_pr.default_branch()}.  "
                 + "This usually happens because there is a non ghstack change in the PR.  "
                 + f"Please sync them and try again (ex. make the changes on {orig_ref} and run ghstack)."
             )