rtd: Compare against the merge-base rather than main

Author: zware

.readthedocs.yml

@@ -20,7 +20,7 @@ build:
     # If there are no changes (git diff exits with 0) we force the command to return with 183.
     # This is a special exit code on Read the Docs that will cancel the build immediately.
     - |
-      if [ "$READTHEDOCS_VERSION_TYPE" = "external" ] && [ "$(git diff --quiet origin/main -- Doc/ .readthedocs.yml; echo $?)" -eq 0 ];
+      if [ "$READTHEDOCS_VERSION_TYPE" = "external" ] && [ "$(git diff --quiet $(git merge-base HEAD origin/main) -- Doc/ .readthedocs.yml; echo $?)" -eq 0 ];
       then
         echo "No changes to Doc/ - exiting the build.";
         exit 183;