written but not run - merge a single file

git-merge-single.sh

#!/bin/bash
TEMP="$(mktemp --tmpdir -d 'git-merge-single.XXXXXXXXXX')"
[ -n "$TEMP" -a -d "$TEMP" -a -w "$TEMP" ] || exit 1
_cleanup(){ rm -rf "$TEMP"; }
trap _cleanup EXIT

their_commit="${1%%:*}"
file="${1#*:}"

git cat-file blob "$their_commit:./$file" > "$TEMP/theirs"
git cat-file blob "$(git merge-base HEAD $their_commit):./$file" > "$TEMP/base"

git merge-file "./$file" "$TEMP/base" "$TEMP/theirs"