Wait for github to update PR after a force push

Summary:
This is trying to fix the exact same issue as described in D57741395. Except the mentioned diff failed to fully close the race.

This diff fully closes the race by inserting a loop that waits for GH to acknowledge the force push.

I've checked that the SHAs returned by the two libraries are indeed comparable:
```
>>> import github
>>> g = github.Github()
>>> repo = g.get_repo("PyGithub/PyGithub")
>>> pr = repo.get_pull(2987)
>>> pr.head.sha
'294e68a3c605a58695c29e0982a620489956423e'

>>> from git import Repo
>>> r = Repo("~/dev/linux")
>>> r.head.commit.hexsha
'911edc69c832161b62a8ad10a6972290157a7bd3'
```

Reviewed By: anakryiko

Differential Revision: D58542702

fbshipit-source-id: e446828c405aa4edb9b702f00bbd2c9f3671a7ce

kernel_patches_daemon/branch_worker.py

@@ -908,10 +908,21 @@ async def apply_push_comment(
                 branch_name=branch_name,
                 can_create=True,
             )
+            assert pr
             self.repo_local.git.push("--force", "origin", branch_name)
+
             # Metadata inside `pr` may be stale from the force push; refresh it
-            if pr:
+            pr.update()
+            wanted_sha = self.repo_local.head.commit.hexsha
+            for _ in range(30):
+                if pr.head.sha == wanted_sha:
+                    break
+                logger.info(f"Waiting for {pr} sha={pr.head.sha} to go to {wanted_sha}")
+                await asyncio.sleep(1)
                 pr.update()
+            else:
+                raise RuntimeError("Github failed to update PR after force push")
+
             return pr
         # we don't have a branch, also means no PR, push first then create PR
         elif branch_name not in self.branches: