Use sparse checkout

[dguest]

I finally did a bit of a rewrite of the internals here to take advantage of sparse-checkout being a standard git command since 2020.

A lot of things work better as a result:

• Less weird (rather fragile seeming) text manipulation required
• The initial checkout doesn't leave git in a state where status marks every file in Athena as deleted
• I fixed some ambiguity with get-fatlas-add packages: if the package name doesn't return a unique match the command fails.
• It's easier to debug the add tab-completion: if you set the TABTEST variable and then follow it in another terminal via tail -f $TABTEST, you can get some info on what the complete function is trying to do.

I'll leave this open for a few days in case anyone happens to be following this repo and spots any issues.