Return to Answer

tag ${size}\0object ${sha_of_commit_it-points-to}
type ${type_of_object_it_points_to}
tag ${name_of_tag}
tagger ${username_and_email} ${seconds_since_utc} ${timezone}

${commit_message_including_gpg}

tag ${size}\0object ${sha_of_object_it-points-to}
type ${type_of_object_it_points_to}
tag ${name_of_tag}
tagger ${username_and_email} ${seconds_since_utc} ${timezone}

${commit_message_including_gpg}

If the tag points to a commit object, then sha_of_object_it-points-to is the SHA of the commit. Note that a tag can also point to non-commit objects like blobs, though this is not very commonly used.

Deduce the format and generate a new tag

This generates a git repo from scratch, including a tag:

Reverse engineer the format from a minimal example

To ensure that we understood it correctly, here is a Python script that generates a working git repo from scratch, including a tag:

Tested on git 2.16.1, Ubuntu 18.04.