Add shell script for performing atomic pushes across branches (#50401)

dev-tools/atomic_push.sh

@@ -0,0 +1,55 @@
+#!/usr/bin/env bash
+set -e
+
+if [ "$#" -eq 0 ]; then
+    printf 'Usage: %s <origin> <branch> <branch> ...\n' "$(basename "$0")"
+    exit 0;
+fi
+
+REMOTE="$1"
+
+if ! git ls-remote --exit-code "${REMOTE}" > /dev/null 2>&1; then
+    echo >&2 "Remote '${REMOTE}' is not valid."
+    exit 1;
+fi
+
+if [ "$#" -lt 3 ]; then
+    echo >&2 "You must specify at least two branchs to push."
+    exit 1;
+fi
+
+if ! git diff-index --quiet HEAD -- ; then
+    echo >&2 "Found uncommitted changes in working copy."
+    exit 1;
+fi
+
+ATOMIC_COMMIT_DATE="$(date)"
+
+for BRANCH in "${@:2}"
+do
+    echo "Validating branch '${BRANCH}'..."
+    
+    # Vailidate that script arguments are valid local branch names
+    if ! git show-ref --verify --quiet "refs/heads/${BRANCH}"; then
+        echo >&2 "No such branch named '${BRANCH}'."
+        exit 1;
+    fi
+    
+    # Pull and rebase all branches to ensure we've incorporated any new upstream commits
+    git checkout --quiet ${BRANCH}
+    git pull "${REMOTE}" --rebase --quiet
+    
+    PENDING_COMMITS=$(git log ${REMOTE}/${BRANCH}..HEAD --oneline | grep "^.*$" -c || true)
+    
+    # Ensure that there is exactly 1 unpushed commit in the branch
+    if [ "${PENDING_COMMITS}" -ne 1 ]; then
+        echo >&2 "Expected exactly 1 pending commit for branch '${BRANCH}' but ${PENDING_COMMITS} exist."
+        exit 1;
+    fi
+    
+    # Amend HEAD commit to ensure all branch commit dates are the same
+    GIT_COMMITTER_DATE="${ATOMIC_COMMIT_DATE}" git commit --amend --no-edit --quiet
+done
+
+echo "Pushing to remote '${REMOTE}'..."
+git push --atomic "${REMOTE}" "${@:2}"