How do I pass username and password while using Ansible Git module?

You can use something like this:

---
- hosts: all 
  gather_facts: no
  become: yes
  tasks:
    - name: install git package
      apt:
        name: git

    - name: Get updated files from git repository 
      git: 
        repo: "https://{{ githubuser | urlencode }}:{{ githubpassword | urlencode }}@github.com/privrepo.git"
        dest: /tmp

Note: {{ githubpassword | urlencode }} is used here, if your password also contains special characters @,#,$ etc

Then execute the following playbook:

ansible-playbook -i hosts github.yml -e "githubuser=arbabname" -e "githubpassword=xxxxxxx"

Note: Make sure you put the credentials in ansible vaults or pass it
secure way