src/Composer/Repository/Vcs/GitHubDriver.php
protected function fetchRootIdentifier()
    {
        if ($this->repoData) {
            return;
        }

        $repoDataUrl = $this->getApiUrl() . '/repos/'.$this->owner.'/'.$this->repository;

        $this->repoData = JsonFile::parseJson($this->getContents($repoDataUrl, true), $repoDataUrl);
        if (null === $this->repoData && null !== $this->gitDriver) {
            return;
        }

        $this->owner = $this->repoData['owner']['login'];
        $this->repository = $this->repoData['name'];

        $this->isPrivate = !empty($this->repoData['private']);
        if (isset($this->repoData['default_branch'])) {
            $this->rootIdentifier = $this->repoData['default_branch'];
        } elseif (isset($this->repoData['master_branch'])) {
            $this->rootIdentifier = $this->repoData['master_branch'];
        } else {
            $this->rootIdentifier = 'master';
        }
        $this->hasIssues = !empty($this->repoData['has_issues']);
    }
protected function fetchRootIdentifier()
    {
        if ($this->repoData) {
            return;
        }

        $repoDataUrl = $this->getApiUrl() . '/repos/'.$this->owner.'/'.$this->repository;

        $this->repoData = JsonFile::parseJson($this->getContents($repoDataUrl, true), $repoDataUrl);
        if (null === $this->repoData && null !== $this->gitDriver) {
            return;
        }

        $this->owner = $this->repoData['owner']['login'];
        $this->repository = $this->repoData['name'];

        $this->isPrivate = !empty($this->repoData['private']);
        if (isset($this->repoData['default_branch'])) {
            $this->rootIdentifier = $this->repoData['default_branch'];
        } elseif (isset($this->repoData['master_branch'])) {
            $this->rootIdentifier = $this->repoData['master_branch'];
        } else {
            $this->rootIdentifier = 'master';
        }
        $this->hasIssues = !empty($this->repoData['has_issues']);
    }
Fetch root identifier from GitHub

@throws TransportException