Truncate Git history to after specific SHA1, dropping everything at and before it.

gittruncatehistory.sh

#!/bin/bash -e
# Inspired by: https://github.com/adrienthebo/git-tools/blob/master/git-truncate

if [[ (-z $1) || (-z $2) ]]; then
	echo "Usage: $(basename "$0") DROP_AT_SHA1 BRANCH"
	exit 1
fi

if [[ ! $1 =~ ^[0-9a-f]{7,40}$ ]]; then
	echo "Error: invalid Git commit SHA1" >&2
	exit 1
fi

# note: if fails - include [--force] argument
git filter-branch \
	--parent-filter "sed \"s/-p $1[0-9a-f]*//\"" \
	--prune-empty \
		-- "$2"

IFS=$'\n'
for refName in $(git for-each-ref --format="%(refname)" refs/original); do
	echo "$refName"
	git update-ref -d "$refName"
done

git gc --aggressive --prune=all