Fetch from origin before trying to switch channels (#4053)

diff --git a/.cirrus.yml b/.cirrus.yml
index 91a6562..210a98b 100644
--- a/.cirrus.yml
+++ b/.cirrus.yml
@@ -15,6 +15,11 @@
 
 flutter_upgrade_template: &FLUTTER_UPGRADE_TEMPLATE
   upgrade_flutter_script:
+    # Ensure that the repository has all the branches.
+    - cd $FLUTTER_HOME
+    - git config remote.origin.fetch "+refs/heads/*:refs/remotes/origin/*"
+    - git fetch origin
+    # Switch to the requested branch.
     - flutter channel $CHANNEL
     - flutter upgrade
   << : *TOOL_SETUP_TEMPLATE