GitLab

* Update http to https in Solarized
* Upgrade http to https for 'Burger In Your Shell'
* Upgrade http to https for tmux.github.io

Fixes #4085: core -> plugin dependency issue.
Rename it to git_current_branch for clarity that it's git-specific.
Update all plugins that were calling it to use new name.
Fix variable leaks by making more variables in lib/git.zsh local.
Have lib/git.zsh use [[ ]] instead of [ ] everywhere.

This avoid spawning additional processes, as the $USER variable will
**always** contain the current logged-in user.