fixup! [3.14] Rewrite RTD configuration to use build.jobs rather than build.commands (GH-149429)

no --set-upstream, add explicit origin/?

Author: zware

.readthedocs.yml

@@ -24,7 +24,7 @@ build:
         if [ "$READTHEDOCS_VERSION_TYPE" = "external" ];
         then
           base_branch=3.14;
-          git fetch --set-upstream --depth=50 origin $base_branch;
+          git fetch --depth=50 origin $base_branch:origin/$base_branch;
           for attempt in $(seq 10);
           do
             if ! git merge-base HEAD origin/$base_branch;