Upgrade userland to Debian Buster
Summary:
The updated clang-format package provided for Buster already creates
the clang-format binary.
The completions for Mercurial and Subversion aren't shipped anymore,
and we don't seem to have a current need for them.
This is a follow-up for 8b10c0ea630b.
Test Plan: Build image, run it on Dwellers.
Reviewers: dereckson
Reviewed By: dereckson
Subscribers: Nasqueron Docker deployment squad
Tags: Docker images
Differential Revision: https://devcentral.nasqueron.org/D2345