git-squash-merge

#!/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 "Convert merge commit at HEAD into a single merge commit"
    echo "on top of <onto-commit> (by squashing all commits between HEAD and <onto-commit>)."
    echo
    echo "Syntax: git-squash-merge <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

onto=$1
onto_sha=$(git rev-parse "$onto")

HEAD1=$(git rev-parse HEAD^1)
HEAD2=$(git rev-parse HEAD^2)

if [ $? -ne 0 ]; then
    echo
    echo "ERROR: HEAD is not a merge commit"
    echo
    exit 1
fi

if git rev-parse HEAD^3 >& /dev/null; then
    echo
    echo "ERROR: HEAD has more than 2 parents"
    echo
    exit 1
fi

if [ "$onto_sha" = "$HEAD1" ]; then
    echo
    echo "ERROR: '$onto' already (first) parent of HEAD."
    echo "       No commits to squash."
    echo
    exit 1
fi
if [ "$onto_sha" = "$HEAD2" ]; then
    echo
    echo "ERROR: '$onto' already (second) parent of HEAD."
    echo "       No commits to squash."
    echo
    exit 1
fi

HEAD_MSG=$(git log -1 --format="%B" HEAD)

if git merge-base --is-ancestor $onto $HEAD1; then
    if git merge-base --is-ancestor $onto $HEAD2; then
        echo
	echo "ERROR: '$onto' is reachable from both HEAD^1 and HEAD^2."
	echo "       Don't know what to do."
	echo
	exit 1
    fi

    SQUASH_LOG=$(git log --format="%n%H %an%n%B" --ancestry-path $onto..$HEAD1)

    #git-create-merge $onto $HEAD2
    run git reset --soft $onto
    run git update-ref MERGE_HEAD $HEAD2
    git commit --edit -m "$HEAD_MSG" -m "SQUASHED commits:" -m "$SQUASH_LOG"

elif git merge-base --is-ancestor $onto $HEAD2; then

    SQUASH_LOG=$(git log --format="%n%H %an%n%B" --ancestry-path $onto..$HEAD2)

    #git-create-merge $HEAD1 $onto
    run git reset --soft $HEAD1
    run git update-ref MERGE_HEAD $onto
    git commit --edit -m "$HEAD_MSG" -m "SQUASHED commits:" -m "$SQUASH_LOG"
else
    echo
    echo "ERROR: '$onto' is not reachable from either HEAD^1 or HEAD^2."
    echo
    exit 1
fi