Canonical pngcheck repo at github.com/pnggroup

Hi, Chris, Greg,

TL/DR:
https://github.com/pnggroup/pngcheck
(under construction, but almost finished)

*****

What I did:

I searched online for the old pngcheck archives and I reconstructed a
history with all I could find. If Greg happens to have pngcheck versions
that are even older than 1.99.2, I'd very much like to take them over and
integrate those ones also into the history that I've just built.

See the branch "main.bak":
https://github.com/pnggroup/pngcheck/tree/main.bak

From "main.bak" I reconstructed the "main" branch, which you may also call
"impostor edition" branch if you look carefully and see me impersonating
Greg as "author". My purpose was to preserve the history in the best way
possible, which I did at first with custom tags in the "main.bak" branch,
and then, with a git-filter-branch hack that turned my custom tags into
actual Git fields. Various Git importers (like Mercurial-to-Git or
SVN-to-Git) use a similar trick to fill in the author fields also.

I added tags as well, which I signed with my own GPG signature that I'm
using for libpng as well. However, if you, Greg, would prefer to sign those
tags instead of me, please tell me so, and I'll remove my tags.

*****

What I would like to do:

With the clarification that I still intend to force-push commits until the
public announcement, I would like to ask Chris if he would like to transfer
his repo from his account (github.com/svgeesus/pngcheck) over to pnggroup.
(I did the same with github.com/glennrp/libpng BTW.) Doing so wouldn't be
strictly necessary, but it will bring forth the nice-to-have history of
recent development.

(Chris, you should have admin permissions to github.com/pnggroup, and you
should be able to rename github.com/pnggroup/pngcheck or delete it
altogether before you transfer your own, but if you can't do it for any
reason, please let me know. Everything at github.com/pnggroup/pngcheck is
mirrored at github.com/ctruta/pngcheck, so, no worries about losing stuff.)

Another decision that I made, but please tell me if you wanted it done
differently, was to do a rebase of Chris' commits on top of the history
that I've built from Greg's tarballs. That means the loss of Chris' history
of merges. If Chris wants to preserve that as well, then I can do an
"octopus merge", then force-push that one over.

*****

When I created github.com/pnggroup I was envisioning the bringup of all
past and present projects that we've done in our group. So pngcheck is the
first to come along. I can hardly wait to finish it up and send a public
announcement!

Sincerely,
Cosmin