"git maintenance start" adds entry to the main config file, not to already existing maintenance section in included file

Hello!

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

I've a git config (in ~/.config/git/config) which reads:

```ini
...
[include]
    path = git_config_local_maintenance
....
```

In ~/.config/git/git_config_local_maintenance I have:

```ini
[maintenance]
    repo =/User/jankatins/projects/whatever
```

ran `git maintenance start` in a new repo.

What did you expect to happen? (Expected behavior)

That the new repo gets appended in ~/.config/git/git_config_local_maintenance to the already existing maintenance section.

What happened instead? (Actual behavior)

An additional maintenance section was added in the main config file with the repo key added there.

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

The additional maintenance section in the main config file instead of appending the repo in the already existing section in the included file.

This is "undesired", as I have my main git config file in a dotfile manager, but don't want to have the (much more frequently changing and laptop specific) maintenance entries in the dotfile repo.

``` [System Info] git version: git version 2.44.0 cpu: x86_64 sizeof-long: 8 sizeof-size_t: 8 shell-path: /bin/sh feature: fsmonitor--daemon uname: Darwin 23.4.0 Darwin Kernel Version 23.4.0: Wed Feb 21 21:44:31 PST 2024; root:xnu-10063.101.15~2/RELEASE_X86_64 x86_64 compiler info: clang: 15.0.0 (clang-1500.1.0.2.5) libc info: no libc information available $SHELL (typically, interactive shell): /usr/local/bin/zsh ```

Kind regards

Jan -- jasc@gmx.net