How do I clone git repo using plugin code.

The method that you refer to is a static method that you can call from everywhere.

You also have to make sure, that in your Gradle configuration, you've added a dependency to the git4idea plugin:

intellij {
    // ...
    plugins 'java', 'git4idea'
}

Can you provide me with any document on how to use Git API ? As I'm looking not to handle user credentials for my plugin separately.

Git plugin sources are available in JetBrains GitHub repository: https://github.com/JetBrains/intellij-community/tree/master/plugins/git4idea/src/git4idea

Probably you'll be interested in GitHttpGuiAuthenticator class, however, you should check the sources first which have JavaDocs describing most of the plugin functionalities.

I am still getting this same error as before 

Caused by: java.lang.ClassNotFoundException: git4idea.commands.Git PluginClassLoader[XX, 0.1-SNAPSHOT] com.intellij.ide.plugins.cl.PluginClassLoader@10841ad2

this is how my gradle file looks like

plugins {
    id 'org.jetbrains.intellij' version '0.4.16'
    id 'org.jetbrains.kotlin.jvm' version '1.3.61'
}

group 'XX'
version '0.1-SNAPSHOT'

repositories {
    mavenCentral()
}

dependencies {
    implementation "org.jetbrains.kotlin:kotlin-stdlib-jdk8"
}

// See https://github.com/JetBrains/gradle-intellij-plugin/
intellij {
    version '2019.3.3'
    plugins 'git4idea'
}
compileKotlin {
    kotlinOptions.jvmTarget = "1.8"
}
compileTestKotlin {
    kotlinOptions.jvmTarget = "1.8"
}
patchPluginXml {
    changeNotes """
      Add change notes here.<br>
      <em>most HTML tags may be used</em>"""
}

Do you have a Git4Idea plugin dependency set in plugin.xml?

<depends>Git4Idea</depends>

Yeah I had missed that. 
I'm having difficulties finding the right class to clone a repository that is authenticated. 
`GitHttpGuiAuthenticator` is of no help. 

Basically I want this clone method to automatically ask for username and password if required, store it safely and not ask again if the stored pass still works. 

val out = git.clone(
    e.project!!,
    File("/Users/XX/tmp"),
    "https://github.com/testing.git",
    System.currentTimeMillis().toString(),
    object: GitLineHandlerListener{
        override fun onLineAvailable(line: String?, outputType: Key<*>?) {
            println(line)
        }
    }
)