Hi.  I have /usr/lib/git-core on my PATH because I like to be able to
write git-sub-command (which I find helps with tab completion without
the IME-unreliable bash completion system).

AIUI this is an intended and supported mode of operation.  It works
for (nearly) all git commands.  It worked with git-subtree in at least
some previous Debian releases.  But it does not work in trixie.
Looking at the script, it has 4 lines of shell boilerplate already.

For form's sake:

1. Steps

   apt install git
   /usr/lib/git-core/git-subtree

2. Expected output

   Usage message from git-subtree

3. Actual output

   It looks like either your git installation or your
   git-subtree installation is broken.
   [etc]

As a workaround I guess I can put a script called `git-subtree` on my
PATH ahead of /usr/lib/git-core.

Thanks,
Ian.