Fix polkadot companion test (#7732)

substrate/.maintain/gitlab/check_polkadot_companion_build.sh

@@ -45,6 +45,7 @@ cargo install -f --version 0.2.0 diener
 
 # Merge master into our branch before building Polkadot to make sure we don't miss
 # any commits that are required by Polkadot.
+git fetch --depth 20 origin
 git merge origin/master
 
 # Clone the current Polkadot master branch into ./polkadot.