GitLab

* Refactor git module, make Gitea use internal git config, add safe.directory config

* introduce git.InitSimple and git.InitWithConfigSync, make serv cmd use gitconfig

* use HOME instead of GIT_CONFIG_GLOBAL, because git always needs a correct HOME

* fix cmd env in cmd/serv.go

* fine tune error message

* Fix a incorrect test case

* fix configAddNonExist

* fix configAddNonExist logic, add `--fixed-value` flag, add tests

* add configSetNonExist function in case it's needed.

* use configSetNonExist for `user.name` and `user.email`

* add some comments

* Update cmd/serv.go
Co-authored-by: zeripath <art27@cantab.net>

* Update cmd/serv.go
Co-authored-by: zeripath <art27@cantab.net>

* Update modules/git/git.go
Co-authored-by: zeripath <art27@cantab.net>

* Update modules/setting/setting.go
Co-authored-by: zeripath <art27@cantab.net>

* Update modules/git/repo_attribute.go

Co-authored-by: zeri...