upgrade grab

We want git and svn/fcm grabs to allow both a quick, shallow extract and a full checkout with fetch/update.

I think there arebe more grab requirements here, including:
- grabbing just a sub-folder
- filtering, for all grab steps include the folder grab
- ftp
- untar/unzip
- combine: ftp -> untar

todo:
- this ticket is underdefined

No files or tests are named. Start by reviewing the existing git and svn/fcm grab steps and documenting the requirements for shallow extracts versus full checkouts with fetch/update. Done means the scope is agreed and the grab design covers the listed folder, filtering, FTP, archive, and combined-source cases.

Written by the indexing model from the issue text.