How to do a non-fast-forward git merge to a branch that isn't checked out?

Question

I\'m on branch a. I want to merge branch b into branch c. The merge is not a fast-forward, but it also doesn\'t require manual resolut

Answer 1

Additional to the very sophisticated git read-tree answer by @jthill, there is (nowadays) an IMHO easier way doing this utilising git worktree. This is the basic approach:

$ git worktree add /tmp/wt c
$ git -C /tmp/wt merge b
$ git worktree remove /tmp/wt

Below is a little Bash script which you can call like this:

$ worktree-merge c b

Script worktree-merge:

#!/usr/bin/env bash

log() {
    echo -n "LOG: "
    echo "$@" >&2
}

escape() {
    local string="$1"
    echo "${string//[. \/]/-}"
}

cleanup() {
    trap "" SIGINT
    log "Removing temporary worktree '$1' ..."
    git worktree remove --force "$1"
}

prepare_worktree() {
    local reference="$1"
    local worktree="/tmp/MERGE-INTO-`escape "$reference"`"

    log "Creating temporary worktree '$worktree' ..."
    trap "cleanup $worktree" EXIT
    git worktree add --force "$worktree" "$reference"
}

do_merge() {
    local reference="$1"
    local worktree="/tmp/MERGE-INTO-`escape "$reference"`"
    shift

    log "Merging ${@@Q} into ${reference@Q} ..."
    git -C "$worktree" merge "$@"
}

prepare_worktree "$1" &&
do_merge "$@" &&
true