How to access raw git conf from my plugin?

So because at least one person is interested in the solution:
You need to use git4idea
so

Add git4idea to IntelliJ IDEA SDK from:
    .../idea-IC-201.7846.76/plugins/git4idea/lib/git4idea.jar

Add dependency to plugin.xml:
    <depends>Git4Idea</depends>

Code example (Scala)

    val git: Git = Git.getInstance()
    val handler  = new GitLineHandler(project, repoRoot, GitCommand.CONFIG)
    handler.addParameters("user.name")
 val commandResult = git.runCommand(handler)
 commandResult.getOutput // use it

Of source parameters could be any, also more commands are available, not only config

cheers.

If you're using Gradle, you'll also need to add git4idea to your plugins list, like:

intellij {
    plugins = [ 'git4idea' ]
}