You can use this GitHub Action to commit changes made in your workflow run directly to your repo: for example, you use it to lint your code, update documentation, commit updated builds, etc....

This is heavily inspired by git-auto-commit-action (by Stefan Zweifel): that action automatically detects changed files and commits them. While this is useful for most situations, this doesn't commit untracked files and can sometimes commit unintended changes (such as package-lock.json or similar, that may have happened during previous steps).
This action lets you choose the path that you want to use when adding & committing changes so that it works as you would normally do using git on your machine.

Usage

Add a step like this to your workflow:

- uses: EndBug/add-and-commit@v4 # You can change this to use a specific version
  with:
    # The arguments for the `git add` command (see the paragraph below for more info)
    # Default: '.'
    add: 'src'

    # The name of the user that will be displayed as the author of the commit
    # Default: author of the commit that triggered the run
    author_name: Your Name

    # The email of the user that will be displayed as the author of the commit
    # Default: author of the commit that triggered the run
    author_email: mail@example.com

    # The local path to the directory where your repository is located. You should use actions/checkout first to set it up
    # Default: '.'
    cwd: './path/to/the/repo'

    # Whether to use the --force option on `git add`, in order to bypass eventual gitignores
    # Default: false
    force: true

    # The message for the commit
    # Default: 'Commit from GitHub Actions'
    message: 'Your commit message'

    # Name of the branch to use, if different from the one that triggered the workflow
    # Default: the branch that triggered the workflow (from GITHUB_REF)
    ref: 'someOtherBranch'

    # The arguments for the `git rm` command (see the paragraph below for more info)
    # Default: ''
    remove: "./dir/old_file.js"

    # Name of the tag to add to the new commit. Use an empty string to disable tagging.
    # Default: ''
    tag: "v1.0.0"

  env:
    # This is necessary in order to push a commit to the repo
    GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }} # Leave this line unchanged

Environment variables:

The only env variable required is the token for the action to run: GitHub generates one automatically, but you need to pass it through env to make it available to actions. You can find more about GITHUB_TOKEN here.
That said, you can just copy the example line and not worry about it. If you do want to use a different token you can pass that in, but I wouldn't see any possible advantage in doing so.

Adding files:

The action adds files using a regular git add command, so you can put every kind of argument in the add option. For example, if you don't want it to use a recursive behavior: $(find . -maxdepth 1 -name *.js). The script will not stop if one of the git commands fails. E.g.: if your command shows a "fatal: pathspec 'yourFile' did not match any files" error the action will go on.

Deleting files:

You can delete files with the remove option: that runs a git rm command that will stage the files in the given path for removal. The script will not stop if one of the git commands fails. E.g.: if your command shows a "fatal: pathspec 'yourFile' did not match any files" error the action will go on.

Examples:

Do you want to lint your JavaScript files, located in the src folder, with ESLint, so that fixable changes are done without your intervention? You can use a workflow like this:

name: Lint source code
on: push

jobs: 
  run:
    name: Lint with ESLint
    runs-on: ubuntu-latest
    steps: 
    - name: Checkout repo
      uses: actions/checkout@v2

    - name: Set up Node.js
      uses: actions/setup-node@v1
      with:
        node-version: 12.x
    
    - name: Install dependencies
      run: npm install

    - name: Update source code
      run: eslint "src/**" --fix

    - name: Commit changes
      uses: EndBug/add-and-commit@v4
      with:
        author_name: Your Name
        author_email: mail@example.com
        message: "Your commit message"
        add: "*.js"
      env:
        GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}

If you need to run the action on a repository that is not located in $GITHUB_WORKSPACE, you can use the cwd option: the action uses a cd normal command, so the path should follow bash standards.

name: Use a different repository directory
on: push

jobs: 
  run:
    name: Add a text file
    runs-on: ubuntu-latest

    steps:
      # If you need to, you can check out your repo to a different location
      - uses: actions/checkout@v2
        with:
          path: "./pathToRepo/"

      # You can make whatever type of change to the repo...
      - run: echo "123" > ./pathToRepo/file.txt

      # ...and then use the action as you would normally do, but providing the path to the repo
      - uses: EndBug/add-and-commit@v4
        with:
          message: "Add the very useful text file"
          add: "*.txt"
          cwd: "./pathToRepo/"
          force: true
        env:
          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}

License

This action is distributed under the MIT license, check the license for more info.