Bug: Git Maintenance does not register multiple repos

When I run — `git maintenance register` to register a particular repository for maintenance, it appends a new entry to `~/.gitconfig` with the repo name.

```
[maintenance]
       repo = /path/to/repo-1
       repo = /path/to/repo-2
```

Now, since git uses the INI format for these files, this would imply that repo-2 actually overrides repo 1 for maintenance.

This implies that a user can only register a single repository on their system for maintenance.

What did you expect to happen? (Expected behavior) I expected that multiple repositories on the system could be setup for maintenance using this command. Which would mean, the `repo` filed would be an array.

```
[maintenance]
       repo[] = /path/to/repo-1
       repo[] = /path/to/repo-2
```
What happened instead? (Actual behavior)
It appended an entry with the same key, which would mean only the last
key wins, and a user cannot have
multiple repos as a part of maintenance.

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

Anything else you want to add: No.

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 2.39.3 (Apple Git-146) cpu: arm64 no commit associated with this build sizeof-long: 8 sizeof-size_t: 8 shell-path: /bin/sh feature: fsmonitor--daemon uname: Darwin 23.5.0 Darwin Kernel Version 23.5.0: Wed May 1 20:12:58 PDT 2024; root:xnu-10063.121.3~5/RELEASE_ARM64_T6000 arm64 compiler info: clang: 15.0.0 (clang-1500.3.9.4) libc info: no libc information available $SHELL (typically, interactive shell): /opt/homebrew/bin/fish

[Enabled Hooks] post-checkout post-commit post-merge pre-push