feat: implement heroku git:credentials as a git credential helper

[erika-wallace]

Summary

This PR implements heroku git:credentials as a git credential helper for Heroku Git authentication.

When git needs credentials for git.heroku.com, it will call heroku git:credentials get, which reads the stored Heroku auth token and outputs it in the format git expects.

Changes:

• src/commands/git/credentials.ts
  • Implements the git-credential protocol (get, store, erase operations)
  • Reads key=value pairs from stdin — terminated by a blank line
  • Validates protocol=https and host=git.heroku.com before outputting credentials
• src/lib/git/git.ts
  • Added configureCredentialHelper(), which writes !heroku git:credentials as a URL-scoped global git credential helper. Since the helper starts with !, Git executes it as a shell command.
  • Wrapped ux.error() with try/catch in spawn's error handler to route errors through the Promise rejection channel
• src/commands/auth/login.ts
  • Calls configureCredentialHelper() after successful login; silently ignores errors
• src/commands/apps/create.ts, src/commands/git/clone.ts, src/commands/git/remote.ts
  • Each command calls configureCredentialHelper() after git operations to handle the case where git was not installed when heroku login was run

Tests:

• Added tests for git:credentials covering protocol operations and host validation
• Added git credential helper tests to apps:create, git:remote, and git:clone
• Created new test file for auth:login covering login validation and git credential helper configuration
• Updated credentialManagerStub to allow custom return values per test
• Fixed previously skipped tests in test/unit/lib/git/git.unit.test.ts; made execFilePromise accessible as an instance property to properly stub cp.execFile after promisify

Note on git unit tests: The git commands are not well tested, tracked in W-13589124. This PR fixes the skipped tests and adds coverage for the new functionality. Remaining improvements are tracked in that work item.

Type of Change

Breaking Changes (major semver update)

• Add a ! after your change type to denote a change that breaks current behavior

Feature Additions (minor semver update)

• feat: Introduces a new feature to the codebase

Patch Updates (patch semver update)

• fix: Bug fix
• deps: Dependency upgrade
• revert: Revert a previous commit
• chore: Change that does not affect production code
• refactor: Refactoring existing code without changing behavior
• test: Add/update/remove tests

Testing

Notes:

You need access to a Heroku app with a git remote for testing git push heroku main. The test steps will temporarily replace your installed Heroku CLI with the local build and modify your global git config — both are restored in the cleanup steps.

Steps:

1. Pull down this branch
2. heroku logout
3. npm install && npm run build
4. npm link — replaces the installed version of Heroku with this local build
5. restart the terminal
6. which heroku — confirm it shows a local/npm path rather than homebrew
7. heroku login
8. git config --global credential.https://git.heroku.com.helper — should return !heroku git:credentials
9. Edit ~/.netrc and remove any lines for git.heroku.com — this forces git to use the credential helper instead of netrc directly
10. cd into a local app directory with a Heroku remote
11. GIT_TRACE=1 git push heroku main — in the trace output, confirm you see run_command: 'heroku git:credentials get' near the start of authentication
12. Confirm the push completes successfully or "Already up to date"

Cleanup:
13. heroku logout
14. npm unlink -g heroku
15. which heroku — confirm the original Heroku path is restored
16. git config --unset --global credential.https://git.heroku.com.helper
17. heroku login— re-authenticates and writes fresh credentials to ~/.netrc

Screenshots (if applicable)

Related Issues

GUS work item: W-22182925

[tlowrimore-heroku]

LGTM!