Live QA against the beta forge failed its first push with 'Failed to
authenticate user' while the config held a valid token. Cause: credential.helper
is a list accumulated across system/global/repo config, and this machine has
osxkeychain (Xcode gitconfig) plus store (~/.gitconfig). A stale entry for the
forge host answered before our helper.
The same list is a token leak in the other direction: git calls approve on
every helper after a successful auth, so 'store' writes the forge token into
~/.git-credentials in plaintext -- undoing the 0600 config and the
no-token-in-URL rule. Confirmed accidentally during QA when a verification
clone with a URL-embedded token re-created exactly that entry.
Fix: set an empty credential.helper first (git reads that as 'forget the
inherited list'), then add ours -- in install_credential_helper and in the
git clone inside get.
2 regression tests, one of which drives 'git credential fill' against a
poisoned outer helper. 143 tests.