my-uupdate.sh

#!/bin/bash
set -eux
tarball=$1

#tmpdir=$(mktemp -d)
#cleanup () { rm -rf $tmpdir; }
#trap cleanup EXIT
tmpdir=../my-uupdate-work
rm -rf $tmpdir
mkdir $tmpdir

mkdir $tmpdir/unpacked

for tarball; do
    subdir=
    if [ ${tarball#*=} != $tarball ]; then
        subdir=${tarball%=*}
    fi
    mkdir -p $tmpdir/unpacked/$subdir
    tar -C $tmpdir/unpacked/$subdir -xf ${tarball#*=} --strip-components=1
done

index_file=$(realpath .git/my-uupdate-index)
rm -f $index_file
GIT_INDEX_FILE=$index_file GIT_WORK_TREE=$tmpdir/unpacked git add -A -f
tmp_tree=$(GIT_INDEX_FILE=$index_file git write-tree)
git ls-tree ${tmp_tree} > $tmpdir/tree
deb_sha=$(git ls-tree HEAD^{tree} | awk '{ if ($4 == "debian") {  print $3; } }')
echo -e "040000 tree $deb_sha\tdebian" >> $tmpdir/tree
new_tree=$(cat $tmpdir/tree | git mktree)
echo "Update upstream source from '$(for tarball; do echo -n $(basename $tarball); done)'" > $tmpdir/commit-msg
commit=$(git commit-tree -p HEAD -F $tmpdir/commit-msg ${new_tree})
git merge $commit