Set default user.name and user.email

[jablko]

This PR sets up a default user.name and user.email for scripting git commit. Users can continue to configure their own values as now, this PR just affords the option not to.

Shouldn't affect existing users: If users git config --global user.{name,email} before actions/checkout, this will respect existing values. If they git config --global after actions/checkout they'll overwrite these defaults. If they git config --local (before or after), those values take precedence.

Fixes #13, although the issue proposed defaulting to the author/user from the event that triggered the action.

Also discussed in #158, although the PR doesn't propose defaults.

Configuring user.{name,email}/scripting git commit is documented in the README.md. The relevant text of ADR 0153 reads:

Note:

• Users scripting git commit may need to set the username and email. The service does not provide any reasonable default value. Users can add git config user.name <NAME> and git config user.email <EMAIL>. We will document this guidance.

Default user.name

I used $GITHUB_WORKFLOW for the default user.name because it's the most helpful value to have in there? Other proposals include the current README.md value ("github-actions") and the author/user from the event that triggered the action.

Here's how $GITHUB_WORKFLOW looks in GitHub's UI -- however I'm equally happy if a different default is implemented:

Default user.email

I went with the current README.md value ("github-actions@github.com"), however @JojOatXGME lays out the options.

[wheelerlaw]

+1 for using github-actions[bot] <41898282+github-actions[bot]@users.noreply.github.com> as the name and email. That will make it so it shows up nicely in the GitHub UI.