How to write an arbitrary blob to a repository to add “supplemental” information, such as contact information or your public GPG key, but without making that blob part of the standard history.

Feed arbitrary text into git hash-object, and write the corresponding blob to the object store:

echo "Robert P. J. Day" | git hash-object -w --stdin
8a1385ff5d6ab8c195f99931042b7afaf418e6b0
$

$ git show 8a1385ff5d6ab8c195f99931042b7afaf418e6b0
Robert P. J. Day
$

In order for that blob not to be garbage-collected, tag it:

$ git tag fullname 8a1385ff5d6ab8c195f99931042b7afaf418e6b0

Show it:

$ git show fullname
Robert P. J. Day
$

In fact, this technique is used by Git maintainer Junio Hamano to disseminate his public GPG key with the Git source code:

$ git show junio-gpg-pub
tag junio-gpg-pub
Tagger: Junio C Hamano <gitster@pobox.com>
Date:   Thu Nov 20 15:12:15 2014 -0800

GPG key to sign git.git archive

This blob contains the GPG public key I use to sign git.git archive.

To use it to verify tags I signed, do:

  $ git-cat-file blob junio-gpg-pub | gpg --import

to import it into your keyring, and then

  $ git-tag -v $tag_to_be_verified

The fingerprint information for the key is as follows:

$ gpg --fingerprint -k 96AFE6CB\!
pub   4096R/713660A7 2011-10-01
      Key fingerprint = 96E0 7AF2 5771 9559 80DA  D100 20D0 4E5A
      7136 60A7
uid                  Junio C Hamano <gitster@pobox.com>
uid                  Junio C Hamano <junio@pobox.com>
uid                  Junio C Hamano <jch@google.com>
sub   4096R/833262C4 2011-10-01
sub   4096R/96AFE6CB 2011-10-03 [expires: 2015-09-21]
sub   4096R/B3F7CAC9 2014-09-20 [expires: 2017-09-19]
-----BEGIN PGP SIGNATURE-----
Version: GnuPG v1

iQIcBAABAgAGBQJUbnV7AAoJELC16IaWr+bLvjYP+wSOG8rp1y77ExHDJQj7HBLm
9PVQIb70tkiBxAGUpVNNbaoJQBuMAgFdT4Baj8CIo2jdzDeeqbjtcdy/XsHZ3IMl

... etc etc ...