git ssh signing changed broke tag merge message contents

[Linus Torvalds <torvalds@linux-foundation.org>]

So I made the mistake of updating my git tree as I started doing my
merge window for 5.17, and suddenly all the messages from signed tags
disappeared from the merge commits.

I bisected it to commit 02769437e1 ("ssh signing: use sigc struct to
pass payload"), but haven't done any other analysis.

I assume it's the change to fmt-merge-msg.c, but have no time to actually check.

Easy enough to test:

   echo "Dummy file" > dummy
   git commit -m "Dummy commit" dummy
   git tag -s -m "Dummy tag" dummy-tag
   git reset --hard HEAD^
   git merge --no-ff dummy-tag

With the above, you are *supposed* to get a merge message in your
editor something like

    Merge tag 'dummy-tag'

    Dummy tag

    * tag 'dummy-tag':
      Dummy commit

(ok, that last part you only get with merge.summary=true, of course)

But with the broken commit, that "Dummy tag" message from the tag
contents does not exist.

Holler if there are questions, but I'm hoping the above explanation is
clear enough since I'm about to be very busy..

                Linus