Credential config for `git.heroku.com` should reset inherited helpers

The CLI configures a credential helper for `git.heroku.com` as:

```
[credential "https://git.heroku.com"]
helper = !heroku git:credentials
```

Git treats `credential.helper` as an *additive list*, so this appended helper doesn't replace a globally-configured helper — it runs *after* it.

For users with a global helper such as Git Credential Manager (GCM):

```
[credential]
helper =
helper = /usr/local/share/gcm-core/git-credential-manager
```

...GCM is consulted first for `git.heroku.com` which:
1. Causes unnecessary extra work.
2. Could return a stale cached credential, shadowing `heroku git:credentials` and causing `git push heroku` to fail authentication.

Adding a blank `helper =` reset before the CLI's helper scopes the host to the CLI's helper only. A blank value clears helpers inherited so far for this URL context; other URLs are unaffected:

```
[credential "https://git.heroku.com"]
helper =
helper = !heroku git:credentials
```

This mirrors how a global `[credential]` block commonly resets the list before setting its preferred helper. (And is what GCM does in its own config block.)

**Reproduction**
1. Set a global credential helper, e.g. `git config --global credential.helper manager` (GCM).
2. Let the CLI write its `git.heroku.com` credential config.
3. With a stale/incorrect credential cached in GCM for `git.heroku.com`, run `git push heroku` — GCM's credential is used instead of `heroku git:credentials`, and auth fails.

```
$ heroku --version
heroku/11.9.0 darwin-arm64 node-v22.23.2
$ which heroku
/opt/homebrew/bin/heroku
```

Start by locating the CLI code that writes the credential configuration for git.heroku.com, then review how it emits the heroku git:credentials helper. Reproduce the behavior with a global helper and a stale credential, and verify that the generated configuration resets inherited helpers before the CLI helper and that git push heroku uses the CLI credential.

Written by the indexing model from the issue text.