[BUG] replay: segmentation fault when mistyping target to --onto

    $ ./bin-wrappers/git replay --onto="$doesntexist" "$commit"'^!'
    Segmentation fault (core dumped)

I did a bisect starting on current `seen` at a0bdfe7b (Merge branch 'bc/sha1-256-interop-02' into seen, 2025-12-11). That found the “first bad commit” 15cd4ef1 (replay: make atomic ref updates the default behavior, 2025-11-06) (which is on `master`). I started with v2.52.0 as the first known “good”, which I manually checked.

Same segmentation fault on `next` at 674ac2bd (Merge branch 'kh/doc-send-email-paragraph-fix' into next, 2025-12-10).

The following is basically the bisect script except I changed it to make sense outside my own repo.

``` #!/bin/sh

make || exit 125

# Mistyped `seen` for example doesntexist=boh1eixe # Current commit for topic kh/doc-pre-commit-fix commit=8cbbdc92f77a20014d9c425c8b9e4af46e492204

./bin-wrappers/git replay --onto="$doesntexist" "$commit"'^!'

if test $? = 139
then
    exit 1
else
    # Presumably regular failure:
    #     fatal: Replaying down to root commit is not supported yet!
    exit 0
fi
```