Avoid reading .gitignore

By inheritance, we honor [_.gitignore_ files](https://git-scm.com/docs/gitignore) for any directories that contain them.

Ideally, we would not read those files, but use something like .mybaignore instead.

This is presently not possible with pure git.
One option would be to temporarily occlude .gitignore files before our git ops, but this is not liked.

No file, test, or entry point is identified. Start by locating the backup command's Git operations and determine how .gitignore is currently honored; done requires a decided and tested way to use an alternative ignore file without temporarily hiding .gitignore files.

Written by the indexing model from the issue text.