Gradle: Read git commit hash.

build.gradle

def getCheckedOutGitCommitHash() {
    def gitFolder = "$projectDir/.git/"
    def takeFromHash = 12
    /*
     * '.git/HEAD' contains either
     *      in case of detached head: the currently checked out commit hash
     *      otherwise: a reference to a file containing the current commit hash
     */
    def head = new File(gitFolder + "HEAD").text.split(":") // .git/HEAD
    def isCommit = head.length == 1 // e5a7c79edabbf7dd39888442df081b1c9d8e88fd
    // def isRef = head.length > 1     // ref: refs/heads/master

    if(isCommit) return head[0].trim().take(takeFromHash) // e5a7c79edabb

    def refHead = new File(gitFolder + head[1].trim()) // .git/refs/heads/master
    refHead.text.trim().take takeFromHash
}

// You can also use it somewhere else, not just in jar files.
jar {
    // ...
    classifier = getCheckedOutGitCommitHash()
    // ...
}

Googled to this beautiful script, thanks a lot @JonasGroeger!