Fix branch checkout order

Since we're deprecating override-branch in favor of override-checkout,
favor override-checkout when deciding which to use.

Also, change an if to elif to make sure we only do one of these.

Change-Id: I55a94188d9894df27d99416cd3cefff8f3a64718
diff --git a/zuul/executor/server.py b/zuul/executor/server.py
index 759f327..5e3794b 100644
--- a/zuul/executor/server.py
+++ b/zuul/executor/server.py
@@ -768,22 +768,22 @@
         branches = repo.getBranches()
         refs = [r.name for r in repo.getRefs()]
         selected_ref = None
-        if project_override_branch in branches:
-            selected_ref = project_override_branch
-            self.log.info("Checking out %s project override branch %s",
-                          project_name, selected_ref)
         if project_override_checkout in refs:
             selected_ref = project_override_checkout
             self.log.info("Checking out %s project override ref %s",
                           project_name, selected_ref)
-        elif job_override_branch in branches:
-            selected_ref = job_override_branch
-            self.log.info("Checking out %s job override branch %s",
+        elif project_override_branch in branches:
+            selected_ref = project_override_branch
+            self.log.info("Checking out %s project override branch %s",
                           project_name, selected_ref)
         elif job_override_checkout in refs:
             selected_ref = job_override_checkout
             self.log.info("Checking out %s job override ref %s",
                           project_name, selected_ref)
+        elif job_override_branch in branches:
+            selected_ref = job_override_branch
+            self.log.info("Checking out %s job override branch %s",
+                          project_name, selected_ref)
         elif ref and ref.startswith('refs/heads/'):
             selected_ref = ref[len('refs/heads/'):]
             self.log.info("Checking out %s branch ref %s",