composer install --prefer-source不起作用

Question

I have a problem.I want to tell you with code. Problem : composer install --prefer-source

Official Definition :will install the packages from their source which is usually a GitHub or a Subversion repository. In other words, it clones the package's source. In the case where the repository is not found on the vcs, it falls back to the installation from dist.

it is okey.no problem.

my composer.json is

    {
    "name": "root/composer",
    "require": {
        "blabla/url": "dev-master",
        "monolog/monolog": "^1.23"
    },
    "repositories" : [
        {
            "type" : "vcs",
            "url"  : "blabla/url"
        }
    ]
}

this is my composer.json file.The following code is run on terminal

composer install

Cloning repository blabla/url on git is SUCCESSFULL!!!

Packagist for 'blabla/url' is available. but is not same with git repository

the above code worked for git repository.

Now..I change my composer.json file.

{
    "name": "root/composer",
    "require": {
        "blabla/url": "dev-master",
        "monolog/monolog": "^1.23"
    }
}

this is my composer.json file.The following code is run on terminal.

composer install --prefer-source

this code worked for packagist,does not work for git repository

but Do not you need to upload it via git repository?

What is problem?