Support relative paths in git.root

[sghill]

Hi folks!

This adds support for relative paths in git.root in a way that seems recommended from Gradle's docs.

From a quick glance at the implementation it looks like this could be converted to use providerFactory.gradleProperty('git.root') with a convention and mapping to a file instance. I didn't take this step because it's a larger change.

Additional Context

I have a project that attempts to set git.root=.. in a nested directory. It looks like this:

rootDir
    my-gradle-project
        build.gradle
        settings.gradle
        gradle.properties
    other-project

I recently started getting a failure during Gradle's configuration phase that .. must be an absolute path, demonstrated by the included test.