CI: also test the 'next' branch in git.git

Improve on a62c6639 ("ci: Add scheduled jobs testing against Git
master", 2020-09-02) by also testing the 'next' branch in git.git.

This'll give us more advanced warning of any breakages, and the 'next' branch is really quite stable (e.g. Google runs it internally, it's in Debian experimental etc.).

We could also add "seen" here, but that branch is far more likely to produce false positives.

Testing this requires pushing a commit which removes the schedule rule:

diff --git a/.gitlab-ci.yml b/.gitlab-ci.yml
index 9940ca11..9302df43 100644
--- a/.gitlab-ci.yml
+++ b/.gitlab-ci.yml
@@ -175,8 +175,6 @@ test:git_git:
       - GO_VERSION: "1.14"
         GIT_VERSION: ["master", "next"]
         TARGET: [ test, test-with-proxies, test-with-praefect ]
-  rules:
-    - if: '$CI_PIPELINE_SOURCE == "schedule"'

 cover:
   <<: *cache_definition

As can be seen on https://gitlab.com/gitlab-org/gitaly/-/pipelines/219137470 it passes now.

There was an issue where the "next" branch on our mirror wasn't being updated, but as of writing it's what's in github.com/git/git's "next": https://gitlab.com/gitlab-org/gitlab-git/-/tree/next

The "seen" branch is still stale from 4 months ago. Maybe someone kicked or setup a rule for "next", but not "seen": https://gitlab.com/gitlab-org/gitlab-git/-/tree/seen ?