git script to manually associate files in a conflicting merge when rename detection fails

git-merge-associate

#!/bin/bash
#
# Purpose: manually associate missed renames in merge conflicts
#
# Usage: git merge-associate <our-target> <base> <theirs>
#
# Example: After a failed rename detection A/a -> B/b which results
# in CONFLICT (delete/modify) for A/a and corresponding "deleted by us"
# messages in git status, the following invocation can be used to manually
# establish the link:
#
#     git merge-associate B/b :1:A/a :3:A/a
#
# or using a shell with brace expansion:
#
#     git merge-associate B/b :{1,3}:A/a
#
# This registers 
# - :1:A/a as   :1:B/b
# - HEAD:B/b as :2:B/b
# - :3:A/a as   :3:B/b
# and replaces B/b with a merge-conflict version using  "git checkout -m -- B/b".
# 
# After manual resolution of B/b and "git add B/b", A/a can be resolved by 
# "git rm A/a"
#

set -e
#set -x

get_tree()
{
    SPEC="${!1}"
    TREE=""
    
    if [[ -z "$SPEC" ]]; then
        TREE="EMPTY"
    elif [[ "$SPEC" =~ ^:(.): ]]; then
        TREE="INDEX:${BASH_REMATCH[1]}"
    elif [[ "$SPEC" =~ ^: ]]; then
        TREE="INDEX:0"
    elif [[ "$SPEC" =~ ^([^:]+): ]]; then
        TREE=${BASH_REMATCH[1]}
    else # no colon, considered as HEAD
        eval ${1}="HEAD:${!1}"
        TREE="HEAD"
    fi
    eval ${1}_TREE="$TREE"
    eval ${1}_BASENAME="${SPEC##*:}"
}

get_mode_and_sha()
{
    eval SPEC="\${$1}"
    eval TREE="\${$1_TREE}"
    eval BASENAME="\${$1_BASENAME}"
    RESULT_MODE=""
    RESULT_SHA=""
    case "$TREE" in
        EMPTY)
            RESULT_MODE="100644"
            RESULT_SHA="e69de29bb2d1d6434b8b29ae775ad8c2e48c5391"
        ;;
        INDEX:*)
            [[ "$TREE" =~ :(.) ]]
            REQ_STAGE=${BASH_REMATCH[1]}
            while read MODE SHA STAGE FILEPATH; do
                if ! [ "$STAGE" = "$REQ_STAGE" ]; then
                    continue
                fi
                RESULT_MODE="$MODE"
                RESULT_SHA="$SHA"
                break
            done < <(git ls-files --stage "$BASENAME")
        ;;
        *)
            while read MODE TYPE SHA FILEPATH; do
                RESULT_MODE="$MODE"
                RESULT_SHA="$SHA"
            done < <(git ls-tree "$TREE" "$BASENAME")
        ;;
    esac
    if [ -z "$RESULT_MODE" ] ; then
        echo "Could not determine mode for $SPEC"
        exit 1
    fi
    if [ -z "$RESULT_SHA" ] ; then
        echo "Could not determine SHA1 for $SPEC"
        exit 1
    fi
    eval $1_SHA="$RESULT_SHA"
    eval $1_MODE="$RESULT_MODE"
}

if [ "$#" -ne 3 ]; then
    echo "Usage: git merge-associate <our-target> <base> <theirs>"
    exit 1
fi

TARGET="$1"
BASE="$2"
THEIRS="$3"

for VAR in TARGET BASE THEIRS; do
    get_tree $VAR
    get_mode_and_sha $VAR
done

git update-index --index-info <<EOI
000000 0000000000000000000000000000000000000000 0	$TARGET_BASENAME
$BASE_MODE $BASE_SHA 1	$TARGET_BASENAME
$TARGET_MODE $TARGET_SHA 2	$TARGET_BASENAME
$THEIRS_MODE $THEIRS_SHA 3	$TARGET_BASENAME
EOI

git checkout -m -- $TARGET_BASENAME

Does not work with modern git citing:
fatal: malformed index info 000000 0000000000000000000000000000000000000000 0 XXXX/xxx.file