Infection config generator fails to handle multiple ignored directories

[sanmai]

Question	Answer
Infection version	dccbf01 (current master)
Test Framework version	PHPUnit 5.7.27
PHP version	PHP 7.2.9-1
Platform	Linux
Github Repo	https://github.com/pear/OLE

When excluding multiple directories, Infection generates a broken configuration which fails to pass Infection's own checks.

Any directories to exclude from within your source directories?: build
Any directories to exclude from within your source directories?: vendor
Any directories to exclude from within your source directories?: tests
Any directories to exclude from within your source directories?: coverage
Any directories to exclude from within your source directories?: 

Gets you:

"excludes": {
    "0": "vendor",
    "1": "tests",
    "2": "build",
    "5": "coverage"
}

#576 and #577 are related.