User-level git config is ignored after pull request #28792

### What happened?

1. Configure `user.name` and `user.email` at user level (in `~/.gitconfig`).
2. gemini-cli is asked to commit the changes.
3. gemini-cli call `git commit -m "..."` with the bash tool.
4. Get the following error:
```
Author identity unknown

*** Please tell me who you are.

Run

git config --global user.email "you@example.com"
git config --global user.name "Your Name"

to set your account's default identity.
Omit --global to set the identity only in this repository.

fatal: unable to auto-detect email address (got '***@***.(none)')
```

### What did you expect to happen?

git should use the user level config in `~/.gitconfig`, including:
* user name, email, signingKey
* `init.defaultBranch` preference
* user's preference on `pull.rebase`

### Client information

Client Information

Run `gemini` to enter the interactive CLI, then run the `/about` command.

```console
│ About Gemini CLI │
│ │
│ CLI Version 0.57.0 │
│ Git Commit 6b0ae9a6c │
│ Model Auto │
│ Sandbox no sandbox │
│ OS linux │
│ Auth Method Signed in with Google (***) │
│ Tier Gemini Code Assist Standard │
│ GCP Project ***
```

### Login information

_No response_

### Anything else we need to know?

_No response_

No source file or test is named. Reproduce the issue with user.name and user.email in ~/.gitconfig, then trace the bash tool's git commit invocation and its environment/config handling. Done means git commit uses the user-level name, email, signingKey, init.defaultBranch, and pull.rebase preferences.

Written by the indexing model from the issue text.