git-trunchist.sh

#!/bin/sh
# https://stackoverflow.com/a/37105443

fatal() {
        printf '%s\n' "$1"
        exit 1
}

path=
depth=1
commit=

if [ $# -eq 0 ]; then
cat <<- endhelp
Usage: $(basename $0) [options] [path]
Truncate history in git repository at <path> either by depth or by commit.
Options:
-c, --commit COMMIT: shallow since COMMIT (default none, overrides depth if set)
-d, --depth N: shallow to depth N (default 1)
endhelp
exit 1
fi

while [ $# -gt 0 ]; do
        case "$1" in
        '-d'|'--depth')
                depth="$2"
                shift ;;
        '-c'|'--commit')
                commit="$2"
                shift ;;
        * ) path="$1"
        esac
        shift
done

if [ -z "$path" ]; then
        path='.'
fi
path="$( realpath "$path" )"
if ! [ -d "$path" ]; then
        fatal "$path : Not a directory"
fi
if ! [ -w "$path" ]; then
        fatal "'Can't write to $path'"
fi

if ! [ -f "$path/HEAD" -o -d "$path/.git" ]; then
        fatal "$path : Not a git repository"
fi

printf '%s\n' "Repository: $path"

cd "$path"
if [ -n "$commit" ]; then
        git fetch --shallow-since $commit
else
        git fetch --depth=1
fi
git reflog expire --expire-unreachable=now --all
git gc --aggressive --prune=all