Merge checksum before bundle install

Related Issue: gitlab#2932 (closed)

Dependent MR:

• gitlab!1602 (merged)
• !13 (merged)
• gitlab!1623 (merged)

Related Doc: https://docs.gitlab.com/ee/development/gemfile.html#no-gems-fetched-from-git-repositories

Add ./jh/bin/merge_checksum to gitlab step, to merge Upstream Gemfile.checksum and JH additional jh/Gemfile.checksum.jh to JH final checksum file jh/Gemfile.checksum.

---

bundler-checksum init generates the checksum only for Gems of type Bundler::Source::Rubygems (Ignore from path and git).

https://jihulab.com/gitlab-cn/gitlab/blob/7cce1a07b5e01816216b23fe1f10674c233f45de/vendor/gems/bundler-checksum/lib/bundler_checksum/command/init.rb#L21-21

next unless spec.source.is_a?(Bundler::Source::Rubygems)