Prepare for rename of default complement branch (#11971)

use `HEAD` rather than hardcoding `master`

.github/workflows/tests.yml

@@ -345,7 +345,7 @@ jobs:
           path: synapse
 
       # Attempt to check out the same branch of Complement as the PR. If it
-      # doesn't exist, fallback to master.
+      # doesn't exist, fallback to HEAD.
       - name: Checkout complement
         shell: bash
         run: |
@@ -358,8 +358,8 @@ jobs:
           #    for pull requests, otherwise GITHUB_REF).
           # 2. Attempt to use the base branch, e.g. when merging into release-vX.Y
           #    (GITHUB_BASE_REF for pull requests).
-          # 3. Use the default complement branch ("master").
-          for BRANCH_NAME in "$GITHUB_HEAD_REF" "$GITHUB_BASE_REF" "${GITHUB_REF#refs/heads/}" "master"; do
+          # 3. Use the default complement branch ("HEAD").
+          for BRANCH_NAME in "$GITHUB_HEAD_REF" "$GITHUB_BASE_REF" "${GITHUB_REF#refs/heads/}" "HEAD"; do
             # Skip empty branch names and merge commits.
             if [[ -z "$BRANCH_NAME" || $BRANCH_NAME =~ ^refs/pull/.* ]]; then
               continue

changelog.d/11971.misc

+Prepare for rename of default complement branch.