Support --reference to local repository

[thisisnacho]

This is very much a needed feature when dealing with self-hosted runners, caches, monorepos, etc.
Updated #1400 to the latest version of actions/checkout, resolving all conflicts and making it ready to merge.

[lure8225]

Hi any progress here? We would really like to get this into the action for use on self hosted runners

[petergoldsmith-anzx]

Also including our interest in having this for self hosted runners with large repos

[sizehnde]

Bumping this PR. It would greatly benefit certain monorepo workflows.

We have implemented an Action internally to support reference clones that essentially has to reimplement the portions of actions/checkout we care about; it would be nice for this to just be a built in feature.

With the introduction of custom runner images, this is even more desireable now, since this type of workflow can be achieved on GitHub-hosted runners now too, not just self-hosted runners.

[arianvp]

Heya we're using this in production now and it's great. Could this please be merged?

[sizehnde]

I reached out to GitHub support about this PR a few months back and was directed to the "Note" section of the README, which currently says "[...] we are allocating resources towards other areas of Actions and are not taking contributions to this repository at this time." I guess they really mean it!

We have also been using this in our CI system and it has greatly improved checkout performance, both in speed and stability.

[b4u-mw]

Hello,

I have created a more advanced implementation: #2384

Reduces my submodule checkout (5GB) form 2:30min to 30s