BLT-ACSF-3: Preserve gitignore files required by ACSF

[benjifisher]

Fixes acquia/blt-acsf#3

Motivation

The blt artifact:build command removes .gitignore files from the document root. This breaks deployment to ACSF, since the ACSF module specifically requires .gitignore files at docroot/sites/g and docroot/sites/default.

Proposed changes

Exclude the required files from being removed.

Alternatives considered

Testing steps

Prepare a Drupal 9 site for deployment to ACSF:

• Install the packages drupal/acsf, acquia/blt-acsf, and their dependencies.
• Run the commands blt recipes:acsf:init:drush and recipes:acsf:init:all.
• Commit the results to Git.
• Run the command blt artifact:deploy.
• Update your site's code from the ACSF dashboard.

[benjifisher]

I do not expect this PR to be merged in its initial form, since it may have side effects for projects that are not hosted on ACSF. At least, this gives a starting point for finding a more robust solution. Other projects that are using ACSF can apply a patch based on this PR.

[danepowell]

Thanks a lot for the PR, this is the short-term solution I was envisioning as well. I think it's worth merging as-is to unblock ACSF users, but I also want to do more analysis to figure out how to stop this from happening again, and see if there are any better long-term solutions.

Side effects seem unlikely, I don't think .gitignore files in these directories are common, but we'll put a change record in the release notes to be safe.