feat: allow to pass author

This is required to find the stacked pull request if the stack is not
owned by the owner of the token.

This will be leverage to run mergify-cli inside an GitHub action.

Change-Id: I1c850e473aa086afe1f30a194e0da99d29c6bfa9

mergify_cli/__init__.py

@@ -599,12 +599,9 @@ async def get_remote_changes(
     user: str,
     repo: str,
     stack_prefix: str,
+    author: str,
 ) -> RemoteChanges:
-    r_author, r_repo = await asyncio.gather(
-        client.get("/user"),
-        client.get(f"/repos/{user}/{repo}"),
-    )
-    author = r_author.json()["login"]
+    r_repo = await client.get(f"/repos/{user}/{repo}")
     repository = r_repo.json()
 
     r = await client.get(
@@ -660,6 +657,7 @@ async def stack_push(  # noqa: PLR0913, PLR0915, PLR0917
     create_as_draft: bool = False,
     keep_pull_request_title_and_body: bool = False,
     only_update_existing_pulls: bool = False,
+    author: str | None = None,
 ) -> None:
     os.chdir(await git("rev-parse", "--show-toplevel"))
     dest_branch = await git_get_branch_name()
@@ -720,8 +718,18 @@ async def stack_push(  # noqa: PLR0913, PLR0915, PLR0917
         follow_redirects=True,
         timeout=5.0,
     ) as client:
+        if author is None:
+            r_author = await client.get("/user")
+            author = r_author.json()["login"]
+
         with console.status("Retrieving latest pushed stacks"):
-            remote_changes = await get_remote_changes(client, user, repo, stack_prefix)
+            remote_changes = await get_remote_changes(
+                client,
+                user,
+                repo,
+                stack_prefix,
+                author,
+            )
 
         with console.status("Preparing stacked branches..."):
             console.log("Stacked pull request plan:", style="green")
@@ -877,6 +885,7 @@ async def _stack_push(args: argparse.Namespace) -> None:
         args.draft,
         args.keep_pull_request_title_and_body,
         args.only_update_existing_pulls,
+        args.author,
     )
 
 
@@ -951,6 +960,11 @@ async def register_stack_push_parser(
         help="Don't update the title and body of already opened pull requests. "
         "Default fetched from git config if added with `git config --add mergify-cli.stack-keep-pr-title-body true`",
     )
+    parser.add_argument(
+        "--author",
+        help="Set the author of the stack (default: the author of the token)",
+    )
+
     parser.add_argument(
         "--trunk",
         "-t",