git grep --threads 12 --textconv is effectively single-threaded

It looks like the bit of code that is responsible for performing textconv conversions is single-threaded, even if git-grep is provided a number of threads to use.

This greatly limits the usability of textconv filters for grep expressions over a large number of files.

Note the difference in total CPU usage in the following expressions:

$ git grep --threads 12 -e foobar --and -e fizzbuzz &> /dev/null 0.24s user 0.28s system 710% cpu 0.073 total

$ git grep --threads 12 -e foobar --and -e fizzbuzz --textconv &> /dev/null 0.90s user 1.75s system 110% cpu 2.390 total

$ git version git version 2.27.0

Zach Riggle