The Subject tag was turned on over on lists.gnu.org in the mailing
list configuration.  I turned it off.  And actually it needs to be
turned off or it will break DKIM signatures.

The From address I went ahead and unset so the original author's from
address will be used but note that this causes problems depending upon
the SPF, DKIM, and DMARC settings of the sender.  If the sender has a
strict DMARC setting such as Zoho does then this will cause the mail
to be rejected by many email service providers such as Yahoo and
sometimes Google.  This is exactly the same issue as you see where the
mailing list now says Some User via themailinglist now.  That's a
mitigation for this problem.

Note that I did make the requested change.  If it is working for Emacs
then who am I to argue.  But the reason that setting got put in there
was because of a problem elsewhere.  I initially cloned it without
thinking too much about it.  Honestly it has been a year since I have
thought about this aspect of the problem and need to work through it
from the fundamentals up to the top again.

The specific problem is that the the commit diffs are being generated
on the Savannah server and are spoofing the From address of the
committer.  If that is allowed then it works.  If that is not allowed
then mail is rejected.  And perhaps surprisingly it is not the
committer that is unsubscribed but the sites that respect the
commiter's declared settings because they reject the mailing list mail
and eventually Mailman will unsubscribe them due to too many rejects.

Sorry for the delay.  Life had many random convergences preventing me
from having any free time at all.

I looked at things today.  And honestly it took me an embarrassingly
long time to see that the previous post receive hook used "hooks" but
the git-multimail.py hook uses "multimailhook".  That definitely
prevented it from getting any of the configuration.  I failed to see
that was the difference for way too long.  But eventually realized
that was the problem.  Configured it.

And now for real I promise the configuration for auctex is identical
to the configuration for emacs except for the email addresses
configured for mail to be sent.  The next push there will tell if this
is true or not.

> Reading your docs, shouldn't it be mailingList (with capital L)
> instead of mailinglist?

I am pretty sure that on that part of the configuration that case is
ignored.  Therefore any case is equivalent.  That's a definite maybe!
I think.  But that's exactly how it is for the emacs configuration,
which was the one you wanted to clone, and it is working there.

> And maybe commitList would even be more appropriate for the
> auctex-diffs, i.e., one mail with diff per commit instead of one
> diff mail per ref-change/push?

Hold that thought.  Let's get things working first.  Just like it is
with emacs.  And then can make that modification and see what it
looksl like.

> The docs for mailingList are a bit vague here but I would prefer one
> mail to auctex-diffs per commit instead of one mail with 5 diffs when
> I push 5 commits in one go.

There are projects that have asked to disable the summary message.
Here is an example.

[multimailhook]
        mailinglist =
        commitList = -email is unavailable-

Producing this result.

https://lists.gnu.org/archive/html/groff-commit/

Reading your docs, shouldn't it be mailingList (with capital L) instead of mailinglist?  And maybe commitList would even be more appropriate for the auctex-diffs, i.e., one mail with diff per commit instead of one diff mail per ref-change/push?

The docs for mailingList are a bit vague here but I would prefer one mail to auctex-diffs per commit instead of one mail with 5 diffs when I push 5 commits in one go.

Good IRC ping.  It had slipped off my active to-do list.

The emacs hook uses a different program for emailing out the commit
notifications and the configuration is significantly different.  I
have cloned the emacs git commit diff configuration to auctex and then
modified it.  The previous hook contained this comment.

# The auctex group would like both short commit email to
# -email is unavailable- and longer full diff emails to
# -email is unavailable- list.  This means multiple post-receive
# scripts and two email destination addresses.

[hooks]
        mailinglist = -email is unavailable-
        mailinglist2 = -email is unavailable-

Therefore I configured it this following way.

[hooks]
        mailinglist = -email is unavailable-
        refchangeList = -email is unavailable-

Where the following documentation applies.

multimailhook.mailingList
    The list of email addresses to which notification emails should be
    sent, as RFC 2822 email addresses separated by commas.  This
    configuration option can be multivalued.  Leave it unset or set it
    to the empty string to not send emails by default.  The next few
    settings can be used to configure specific address lists for
    specific types of notification email.

multimailhook.refchangeList
    The list of email addresses to which summary emails about
    reference changes should be sent, as RFC 2822 email addresses
    separated by commas.  This configuration option can be
    multivalued.  The default is the value in
    multimailhook.mailingList.  Set this value to "none" (or the empty
    string) to prevent reference change emails from being sent even if
    multimailhook.mailingList is set.

multimailhook.announceList
    The list of email addresses to which emails about new annotated
    tags should be sent, as RFC 2822 email addresses separated by
    commas.  This configuration option can be multivalued.  The
    default is the value in multimailhook.refchangeList or
    multimailhook.mailingList.  Set this value to "none" (or the empty
    string) to prevent annotated tag announcement emails from being sent
    even if one of the other values is set.

multimailhook.commitList
    The list of email addresses to which emails about individual new
    commits should be sent, as RFC 2822 email addresses separated by
    commas.  This configuration option can be multivalued.  The
    default is the value in multimailhook.mailingList.  Set this value
    to "none" (or the empty string) to prevent notification emails about
    individual commits from being sent even if
    multimailhook.mailingList is set.

git-multimail upstream is located at

https://github.com/git-multimail/git-multimail

I think that sets things up as requested.  Feel free to ask for
modifications to what I have selected.  I simply took my best guess at
what would be the new desired configuration.  Happy to tweak this
around as you wish.  Note that I am not a python person and so hacking
around on the program is best done by others.

It's too late for me tonight to fix this but I wanted to acknowledge the problem report.

As to why these migh be different it is because over time the best way to configure these changed.  Initially there was one way.  Then there was another way.  And then there was modifications.  And therefore the explanation is that they are different.  However it will be simple to sync them up and update auctex to match emacs.