Add script for syncing subtree

.gitignore

@@ -1 +1,2 @@
 /target
+git-subtree.sh

subtree-sync.sh

@@ -0,0 +1,52 @@
+#!/bin/bash
+
+set -eou pipefail
+
+git fetch origin
+pushd $2
+git fetch origin
+popd
+
+if [ "$(git rev-parse --show-prefix)" != "" ]; then
+    echo "Run this script from the git root" >&2
+    exit 1
+fi
+
+if [ "$(git rev-parse HEAD)" != "$(git rev-parse origin/master)" ]; then
+    echo "$(pwd) is not at origin/master" >&2
+    exit 1
+fi
+
+if [ ! -f library/portable-simd/git-subtree.sh ]; then
+    curl -sS https://raw.githubusercontent.com/bjorn3/git/tqc-subtree-portable/contrib/subtree/git-subtree.sh -o library/portable-simd/git-subtree.sh
+    chmod +x library/portable-simd/git-subtree.sh
+fi
+
+today=$(date +%Y-%m-%d)
+
+case $1 in
+    "push")
+        upstream=rust-upstream-$today
+        merge=sync-from-rust-$today
+
+        pushd $2
+        git checkout master
+        git pull
+        popd
+
+        library/portable-simd/git-subtree.sh push -P library/portable-simd $2 $upstream
+
+        pushd $2
+        git checkout -B $merge origin/master
+        git merge $upstream
+        popd
+        echo "Branch \`$merge\` created in \`$2\`. You may need to resolve merge conflicts."
+        ;;
+    "pull")
+        branch=sync-from-portable-simd-$today
+
+        git checkout -B $branch
+        echo "Creating branch \`$branch\`... You may need to resolve merge conflicts."
+        library/portable-simd/git-subtree.sh pull -P library/portable-simd $2 origin/master
+        ;;
+esac