Use git branch -D for reference_branch

This commit allows the use of a reference_branch that is not merged into
the current working directory.

Imports have been sorted by isort

update-zones.py

@@ -5,9 +5,9 @@
 import argparse
 import os
 import tempfile
-from subprocess import CalledProcessError, run
-from sys import stderr
 from email.message import EmailMessage, MIMEPart
+from subprocess import DEVNULL, CalledProcessError, run
+from sys import stderr
 
 REFERENCE_BRANCH = "provisioned"
 DEFAULT_BRANCH = "origin/main"
@@ -246,7 +246,12 @@ def main(reference_branch, target_branch, audit_email_address, audit_sender_addr
 
     changelog = get_changelog(reference_branch, target_branch)
 
-    run(["git", "branch", "-d", reference_branch], check=True)
+    run(
+        ["git", "branch", "-D", reference_branch],
+        check=True,
+        stdout=DEVNULL,
+        stderr=DEVNULL,
+    )
     run(["git", "branch", reference_branch, target_branch], check=True)
 
     send_audit_mail(diffs, audit_email_address, audit_sender_address, changelog)