[BUG] rebase: can write reflog with uninit. `action` string

From: code@khaugsbakk.name

We did `git rebase --rebase-merges` on a branch with merge commits. Including back merges. The reflog after that showed some weird symbols for certain merge commits (only merges):

    e9c962f2ea0 HEAD@{8}: <binary>�: Merged in <branch> (pull request #4441)

Some merge commits were normal.

No backmerges were affected.

We have a main branch. This other branch was created from the main branch. It had been kept up to date with backmerges. Then someone did a rebase on it once it was supposed to go into the main branch soonish.

It looks like the string is uninit.  The values are different each time.

§ Bisection

Bisects to d188a60d722 (sequencer: stop exporting GIT_REFLOG_ACTION, 2022-11-09).

§ Reproduction on latest code

Reproduced on `master`, on f65182a99e5 (The ninth batch, 2025-04-24).

§ Gdb

This is the backtrace when I get the apparently uninit. string:

```
#0  run_git_commit (defmsg=0x555555babe70 "<repo path>/MERGE_MSG", opts=0x7fffffffc070, flags=0) at sequencer.c:1158
#1  0x00005555558740d9 in do_merge (r=0x555555a67020 <the_repo>, commit=0x555555b75cb0,
    arg=0x555555b2164d "<branch stuff> # Merged in <branch> { (pull request #4464)\nlabel branch-point-9\npick 4026b5ced849724bd3857283b6ad50c8609b6d33 only sh"..., arg_len=125, flags=0, check_todo=0x7fffffffb1e0, opts=0x7fffffffc070) at sequencer.c:4380
#2  0x0000555555876629 in pick_commits (r=0x555555a67020 <the_repo>, todo_list=0x7fffffffbf50, opts=0x7fffffffc070) at sequencer.c:5048
#3  0x0000555555877eeb in sequencer_continue (r=0x555555a67020 <the_repo>, opts=0x7fffffffc070) at sequencer.c:5480
#4  0x000055555563a491 in run_sequencer_rebase (opts=0x7fffffffc330) at builtin/rebase.c:369
#5  0x000055555563bc74 in run_specific_rebase (opts=0x7fffffffc330) at builtin/rebase.c:746
#6  0x000055555563fe2a in cmd_rebase (argc=0, argv=0x555555a73890, prefix=0x0, repo=0x555555a67020 <the_repo>) at builtin/rebase.c:1878
#7  0x0000555555574c0d in run_builtin (p=0x555555a34908 <commands+2280>, argc=2, argv=0x555555a73890, repo=0x555555a67020 <the_repo>) at git.c:480
#8  0x00005555555750ca in handle_builtin (args=0x7fffffffd8a0) at git.c:743
#9  0x000055555557538c in run_argv (args=0x7fffffffd8a0) at git.c:810
#10 0x00005555555759e2 in cmd_main (argc=2, argv=0x7fffffffda30) at git.c:950
#11 0x000055555569b0c3 in main (argc=5, argv=0x7fffffffda18) at common-main.c:9
```

§ No reproduction script

I was unable to reproduce with a simple repo. setup.  I tried:

1. Creating a side branch which had a merge
2. The side branch conflicted with the other branch
3. Rebased with `--rebase-merges`

-----

Normal bugreport questionaire follows.

-----

    Thank you for filling out a Git bug report!
    Please answer the following questions to help us understand your issue.

    What did you do before the bug happened? (Steps to reproduce your issue)

`git rebase --rebase-merges` on a branch with merge commits. Including back merges.

    What did you expect to happen? (Expected behavior)

Normal “action” string for the reflog like for example `continue`:

    3f90f6ab14d (HEAD -> <branch>) HEAD@{1}: rebase (continue): Merged in <branch> (pull request #4507)

What happened instead? (Actual behavior)

The “action” (or whatever it is) string is arbitrary bytes. Like some uninit. memory.

``` e9c962f2ea0 HEAD@{8}: <binary>�: Merged in <branch> (pull request #4441) ```

    What's different between what you expected and what actually happened?

Apparently uninit. string.

     Anything else you want to add:

     Please review the rest of the bug report below.
     You can delete any lines you don't wish to share.

[System Info]
git version:
git version 2.49.0.459.gf65182a99e5
cpu: x86_64
built from commit: f65182a99e545d2f2bc22e6c1c2da192133b16a3
sizeof-long: 8
sizeof-size_t: 8
shell-path: /bin/sh
libcurl: 7.81.0
OpenSSL: OpenSSL 3.0.2 15 Mar 2022
zlib: 1.2.11
SHA-1: SHA1_DC
SHA-256: SHA256_BLK
uname: Linux 6.8.0-58-generic #60~22.04.1-Ubuntu SMP PREEMPT_DYNAMIC Fri Mar 28 16:09:21 UTC 2 x86_64
compiler info: gnuc: 11.4
libc info: glibc: 2.35
$SHELL (typically, interactive shell): /bin/bash

[Enabled Hooks]