Fix: [AzurePipelines] don't assume master as target branch when rebasing PRs

azure-pipelines/templates/ci-git-rebase.yml

@@ -1,9 +1,9 @@
 steps:
-# Rebase to origin/master for every PR. This means users don't have to
-# rebase every time master changes. As long as the PR applies cleanly, we
+# Rebase to target branch for every PR. This means users don't have to
+# rebase every time target branch changes. As long as the PR applies cleanly, we
 # will validate it.
-- script: |
+- bash: |
     git config user.email 'info@openttd.org'
     git config user.name 'OpenTTD CI'
-    git rebase origin/master
-  displayName: 'Rebase to master'
+    git rebase origin/$SYSTEM_PULLREQUEST_TARGETBRANCH
+  displayName: 'Rebase to $(System.PullRequest.TargetBranch)'