Default branch checkout option

[alexanderkranga]

Hi,

This change will allow to checkout the default repository branch if the requested ref does not exist. We have been using this in our fork successfully for more than a year. We can see that this feature was requested long time ago, so we would like to contribute.

This fixes https://github.com/actions/checkout/issues/512

Thank you!

[alexanderkranga]

how can we move this forward?

[sergeykranga]

can we get the outstanding review done here? thanks!

[alexandrepa]

Would love to have this feature release in next version 👍