Hello Nico,

Amin Bandali alerted me to the issues here.  I am one of the Savannah
Hackers and can help with some manual git adjustment on the repository
side of things.

Nicolas Petton wrote:
> I just realized that I pushed master into the emacs-27 branch... I must
> have rebased against master out of habit, like I always do on some other
> git repositories.
> 
> I'm really sorry about this, it has made a big mess in the emacs-27
> branch.
> 
> I thought branches were protected against force-pushing, so I'm not sure
> what is the best way for me to fix my mistake.

Branches are protected against non-fastforward pushes.  Which is a
good thing for preventing loss of data due to accidents involving
forced pushes.  And is a good safety net for times like this too since
we are assured that the repository hasn't lost anything.

This is up to you to decide but I suggest that we manually reset the
emacs-27 branch to the commit hash that it was before this problematic
push.  It's true that for any downstream sandbox that has already
pulled from the branch that they will also need to manually reset
their local sandbox branch too.  But the earlier this is done the more
limited in scope of the users for which will need to take this action.

This is easily done on the superuser side of the repository.  For
paranoia and safety sake I would make a branch of the current emacs-27
branch named emacs-27-oops-2020-07-29 or something for a bit of time
just as a safety backup just in case.  It can be truly deleted later.
Then simply reset the commit of the emacs-27 branch back to the
previous known good one one.  Then further pushes can happen just as
normal on top of it.

The only thing I would need to know is what is the git commit hash to
reset the emacs-27 branch back to in order to be before this push?

Let me know if that is what you would want to do.  Or if there is a
different action you would prefer be taken instead.  We are happy to
help!

Bob