Add git integration

[CyberMango]

Add git integration inside of nvim:

• Files tree marks changed files.
• Easy files diff view.
• Easy file adding/staging.
• Nice commits tree view.
• Thin wrappers for git commands?
• Git blame for code lines.

Keywords to look at:

• fugitive.
• gitgutter.
• vim-signify.
• lazygit.