x/build: add a builder with oldest git supported by cmd/go to catch regressions

[trashhalo]

What did you expect to see?

The bug fix for breaking go on Ubuntu 16 would have a test or ci change to prevent a similar error in the future.

What did you see instead?

A code change with no test change.

Context

#26501 (comment)

[bcmills]

Git 2.7.4 is notable(?) because it is the stock version in Ubuntu 16.04 LTS.¹

However, it's not obvious to me why we should draw the line at Ubuntu 16.04: the Ubuntu project claims to still support 14.04.5 LTS,² and the current release is 18.04 LTS. (Ubuntu 18.04 LTS provides git 2.17,³ which is much more recent.)

I don't actually see a minimum Ubuntu or Git version listed on https://tip.golang.org/doc/install. Perhaps we should add one.

(CC: @rsc @rogpeppe @bradfitz @myitcv)

¹https://packages.ubuntu.com/xenial/vcs/git
²https://wiki.ubuntu.com/Releases
³https://packages.ubuntu.com/bionic/vcs/git

[trashhalo]

I don't actually see a minimum Ubuntu or Git version listed on https://tip.golang.org/doc/install. Perhaps we should add one.

I think thats a wonderful idea!

[myitcv]

Whether Ubuntu 14.04 is a sufficiently broad baseline for this sort of analysis I'm not 100% sure (it's pretty good on the basis it's used by Travis); it ships git version 1.9.1

[bradfitz]

My development VM was Debian Jessie for the longest time and its old git (https://packages.debian.org/jessie/git-core ... 2.1.4) was what finally forced me to update to stretch.

[myitcv]

Note also #26594 (comment), which suggests we might also need to add tests for different remote types.

[bcmills]

A builder with git 2.7.4 in the longtest configuration likely would have caught the regression in #33836.

@toothrot, @dmitshur: any chance we could get an Ubuntu 16.04.6 LTS builder set up for longtest mode? It's still supported by Ubuntu through April 2021.