performs a git-merge and prints a tree-ish without modifying any of your state

git-testmerge.sh

#!/bin/sh
set -e
. "$(git --exec-path)"/git-sh-setup
cleanup() {
    if [ ! -z "$tmp" ]; then
        rm -rf "$tmp"
    fi
}
tmp=""
trap cleanup EXIT
f() {
    tmp=`mktemp -d`
    export GIT_INDEX_FILE="$tmp/i"

    if [ "$#" -eq 2 ]; then
        base=`git merge-base $1 $2`
        git read-tree -m $base $1 $2
        out=`git write-tree 2>/dev/null` && echo $out && return 0
    else
        base=`git rev-parse $1`
    fi
    
    mkdir "$tmp/d"
    export GIT_WORK_TREE="$tmp/d"
    rm -f "$GIT_INDEX_FILE"

    old_git_dir="$GIT_DIR"
    mkdir "$tmp/git"
    export GIT_DIR="$tmp/git"
    for d in objects refs info hooks branches config; do
        ln -s "$old_git_dir/$d" "$GIT_DIR/$d"
    done
    echo "$base" >"$GIT_DIR/HEAD"

    git read-tree $base
    git merge --quiet --no-commit $@  >/dev/null 2>&1 && git write-tree && return 0

    echo "merge failed!" >&2
}   
f "$@"