Cancel stale PR builds on new pushes

OldBuildCanceller was already instantiated but with an empty filter
list, making it a no-op. Populate it to track all builders on any
refs/pull/* branch so that a new push to a PR cancels older pending
or running builds for that same PR, instead of letting them run to
completion alongside the new build.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

Author: alexreinking

master/master.cfg

@@ -29,6 +29,7 @@ from buildbot.steps.shell import SetPropertyFromCommand, ShellCommand
 from buildbot.steps.source.github import GitHub
 from buildbot.steps.worker import MakeDirectory, RemoveDirectory, SetPropertiesFromEnv
 from buildbot.util import httpclientservice
+from buildbot.util.ssfilter import SourceStampFilter
 from buildbot.worker import Worker
 from buildbot.www.auth import UserPasswordAuth
 from buildbot.www.authz import Authz
@@ -1290,7 +1291,10 @@ c["services"] = [
         ],
         verbose=True,
     ),
-    OldBuildCanceller("build_canceller", []),
+    OldBuildCanceller(
+        "build_canceller",
+        [([b.name for b in c["builders"]], SourceStampFilter(branch_re=r"^refs/pull/"))],
+    ),
 ]
 
 # Disable sending usage data