[webkitscmpy] Support specified branch in find_existing_pull_request

https://bugs.webkit.org/show_bug.cgi?id=261246
rdar://115084502

Reviewed by Elliott Williams.

* Tools/Scripts/libraries/webkitscmpy/setup.py: Bump version.
* Tools/Scripts/libraries/webkitscmpy/webkitscmpy/__init__.py: Ditto.
* Tools/Scripts/libraries/webkitscmpy/webkitscmpy/program/pull_request.py:
(PullRequest.find_existing_pull_request): Allow caller to specify branch.

Canonical link: https://commits.webkit.org/268045@main

Tools/Scripts/libraries/webkitscmpy/webkitscmpy/program/pull_request.py

@@ -314,10 +314,11 @@ def pull_request_branch_point(cls, repository, args, **kwargs):
         return branch_point
 
     @classmethod
-    def find_existing_pull_request(cls, repository, remote):
+    def find_existing_pull_request(cls, repository, remote, branch=None):
+        branch = branch or repository.branch
         existing_pr = None
         user, _ = remote.credentials(required=False)
-        for pr in remote.pull_requests.find(opened=None, head=repository.branch):
+        for pr in remote.pull_requests.find(opened=None, head=branch):
             existing_pr = pr
             if not existing_pr.opened:
                 continue