We have a slightly unconventional layout for some of our projects that means the .git folder may not be in the project.rootProject.file('.git') location. Also the code in GitInfoService uses the project path and not the root .git location
def grgit = Grgit.open(currentDir: project.projectDir)
Also in one case we have a git repo where the java project is not at the root directory so we need a way to specify the location of the .git location.