Upgrade Git to 2.40.0

Summary:
Some security issues exist on the current 2020 Git version.

Reference: https://www.cvedetails.com/vulnerability-list/vendor_id-4008/GIT.html

Test Plan:
We eat our own dog food: this change is pushed through
Arcanist image with this patch applied.

Reviewers: dereckson

Reviewed By: dereckson

Subscribers: Nasqueron Docker deployment squad

Tags: Docker images

Differential Revision: https://devcentral.nasqueron.org/D2919