Set upstream

Author: tautschnig

.github/workflows/update-subtree.yml

@@ -78,7 +78,8 @@ jobs:
           echo "Nothing to do, ${UPSTREAM_FROM} matches ${UPSTREAM_HEAD} (${SUBTREE_HEAD_MSG})"
           echo "MERGE_CONFLICTS=noop" >> $GITHUB_ENV
         else
-          git -c user.name=gitbot -c user.email=git@bot rebase origin/subtree/library
+          git branch --set-upstream-to=origin/subtree/library
+          git -c user.name=gitbot -c user.email=git@bot rebase
           echo "MERGE_CONFLICTS=maybe" >> $GITHUB_ENV
         fi