Enable parallel Git checkout in run-task

[ahochheiden]

run-task writes the working tree with git's default single threaded checkout. Since git 2.32, checkout.workers spreads that over several threads, and 0 means one worker per logical core.

Measured on try with a cold git checkout -f of the Firefox tree after a shallow clone, default versus checkout.workers=0:

worker	default	workers=0	change
Linux	27.9s	13.3s	-52%
Windows	111.6s	36.0s	-68%

Proposal: git config --global checkout.workers 0 next to the existing safe.directory write in git_checkout.

[ahochheiden]

I don't seem to have the ability to make this a sub-issue of #865.