add git-annex filter-process

add git-annex filter-process

filter-process: New command that can make git add/checkout faster when
there are a lot of unlocked annexed files or non-annexed files, but that
also makes git add of large annexed files slower.

Use it by running: git
config filter.annex.process 'git-annex filter-process'

Fully tested and working, but I have not benchmarked it at all.
And, incremental hashing is not done when git add uses it, so extra work is
done in that case.

Sponsored-by: Mark Reidenbach on Patreon