git-make-merge-commit

#!/usr/bin/env bash
# Create and checkout a merge commit with exactly 2 parents
# Usage: git make-merge-commit <parent1> <parent2> <message> [treeish=HEAD]

set -e

if [ $# -lt 3 ]; then
    echo "Usage: git make-merge-commit <parent1> <parent2> <message> [treeish=HEAD]" >&2
    exit 1
fi

parent1="$1"
parent2="$2"
message="$3"
treeish="${4:-HEAD}"

# Resolve parents to commit SHAs
parent1_sha=$(git rev-parse "$parent1")
parent2_sha=$(git rev-parse "$parent2")

# Get tree SHA from treeish
if [ "$treeish" = "WORKTREE" ] || [ "$treeish" = "worktree" ]; then
    # Create tree from current worktree state
    tree_sha=$(git write-tree)
else
    # Get tree from specified commit/ref
    tree_sha=$(git rev-parse "$treeish^{tree}")
fi

# Create the merge commit with two parents
commit_sha=$(git commit-tree "$tree_sha" -p "$parent1_sha" -p "$parent2_sha" -m "$message")

# Reset current branch to the new commit
git reset --hard "$commit_sha"

echo "Created merge commit $commit_sha"
echo "Parents: $parent1_sha $parent2_sha"
echo "Tree: $tree_sha"