git-remerge

#!/usr/bin/env bash

run() {
    cmd=
    while [ $# -gt 0 ]; do
	cmd="$cmd $1"
	shift
    done
    echo $cmd
    eval $cmd
    if [ $? -ne 0 ]; then
	echo
	echo "ERROR: '$cmd' failed!"
	echo
	exit 1
    fi
}

if [ "$#" != 1 ]; then
    echo
    echo "Rebase current branch on <onto-commit> while keeping merge commits."
    echo
    echo "Syntax: git-remerge <onto-commit>"
    echo
    exit 1
fi

if ! git rev-parse $1 >& /dev/null; then
    echo
    echo "ERROR: Argument '$1' is not a known commit"
    echo
    exit 1
fi

curr_branch="$(git rev-parse --symbolic-full-name --abbrev-ref HEAD)"

if [ "$curr_branch" = "HEAD" ]; then
    echo "ERROR: Detached head"
    exit 1
fi

onto=$1

if git merge-base --is-ancestor HEAD $onto; then
    echo
    echo "ERROR: '$curr_branch' is already reachable from '$onto'. Nothing to remerge."
    echo
    exit 1
fi

if git merge-base --is-ancestor $onto HEAD; then
    echo
    echo "ERROR: '$onto' is reachable from '$curr_branch'. Nothing to remerge."
    echo
    exit 1
fi

if ! git diff --quiet --exit-code; then
    echo "Cannot remerge: You have unstaged changes."
    echo "Please commit or stash them."
    exit 1
fi
if ! git diff --cached --quiet --exit-code; then
    echo "Cannot remerge: Your index contains uncommitted changes."
    echo "Please commit or stash them."
    exit 1
fi

git branch -D remerge/tmp1 >& /dev/null
git branch -D remerge/tmp2 >& /dev/null
git branch -D remerge/child1 >& /dev/null

#
# Loop down to find the common ancestor
#
parent="$(git rev-parse HEAD)"
old_commits=$parent
while true; do
    echo "Commit to remerge: '$parent'"
    
    child1=$(git rev-parse $parent^1)
    if git merge-base --is-ancestor $child1 $onto; then
	echo "Found remerge base '$child1'"
	break;
    fi
    
    old_commits="$child1 $old_commits"
    parent=$child1
done

#
# Loop up and do new merge commits
#
run git checkout -b remerge/tmp1 $onto

for sha in $old_commits; do
    echo "Remerging commit $sha ..."
    run git merge --no-commit $sha
    run git commit --reuse-message=$sha

    tmp_commits="$tmp_commits $(git rev-parse HEAD)"
done

#
# Loop up through new merge commits and fix children
#
run git checkout -b remerge/tmp2
child1=$onto
for sha in $tmp_commits; do
    echo "Re-childing commit $sha ..."
    run git branch -f remerge/child1 $child1
    run git reset --hard $sha
    run git reset --soft remerge/child1
    if git rev-parse --verify --quiet $sha^2 >/dev/null; then
	# A merge commit. Retain second parent of original merge commit.
	run git update-ref MERGE_HEAD $sha^2^2

	#else A normal commit. Omit second parent
    fi
    run git commit --reuse-message=$sha
    child1="$(git rev-parse HEAD)"
done

run git branch -f $curr_branch
run git checkout $curr_branch

git branch -D remerge/tmp1 >& /dev/null
git branch -D remerge/tmp2 >& /dev/null
git branch -D remerge/child1 >& /dev/null