Look up remote name instead of using origin

misc/scripts/prepare-db-upgrade.sh

@@ -30,7 +30,10 @@ EOF
   exit "${exit_code}"
 }
 
-prev_hash="origin/main"
+# default for prev_hash: the main branch of the remote for 'github/semmle-code'.
+# This works out as a dynamic lookup of the hash of the file in the main branch
+# of the repo.
+prev_hash=$(git remote -v | grep 'github/semmle-code\.git (fetch)$' | cut -f1)/main
 
 while [ $# -gt 0 ]; do
   case "$1" in