GitLab

* clean git support for ver < 2.0

* fine tune tests for markup (which requires git module)

* remove unnecessary comments

* try to fix tests

* try test again

* use const for GitVersionRequired instead of var

* try to fix integration test

* Refactor CheckAttributeReader to make a *git.Repository version

* update document for commit signing with Gitea's internal gitconfig

* update document for commit signing with Gitea's internal gitconfig
Co-authored-by: Andrew Thornton <art27@cantab.net>
Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>