Add rustup pull command

Author: bjorn3

scripts/rustup.sh

@@ -27,14 +27,30 @@ case $1 in
         git commit -m "Rustup to $(rustc -V)"
         ;;
     "push")
-	cg_clif=$(pwd)
-	pushd ../rust
-	branch=update_cg_clif-$(date +%Y-%m-%d)
-	git checkout -b "$branch"
-	git subtree pull --prefix=compiler/rustc_codegen_cranelift/ https://github.com/bjorn3/rustc_codegen_cranelift.git master
-	git push -u my "$branch"
-	popd
+        cg_clif=$(pwd)
+        pushd ../rust
+        git pull origin master
+        branch=sync_cg_clif-$(date +%Y-%m-%d)
+        git checkout -b "$branch"
+        git subtree pull --prefix=compiler/rustc_codegen_cranelift/ https://github.com/bjorn3/rustc_codegen_cranelift.git master
+        git push -u my "$branch"
+
+        # immediately merge the merge commit into cg_clif to prevent merge conflicts when syncing
+        # from rust-lang/rust later
+        git subtree push --prefix=compiler/rustc_codegen_cranelift/ "$cg_clif" sync_from_rust
+        popd
+        git merge sync_from_rust
 	;;
+    "pull")
+        cg_clif=$(pwd)
+        pushd ../rust
+        git pull origin master
+        rust_vers="$(git rev-parse HEAD)"
+        git subtree push --prefix=compiler/rustc_codegen_cranelift/ "$cg_clif" sync_from_rust
+        popd
+        git merge sync_from_rust -m "Sync from rust $rust_vers"
+        git branch -d sync_from_rust
+        ;;
     *)
         echo "Unknown command '$1'"
         echo "Usage: ./rustup.sh prepare|commit"