git: use neogit tab workflow

[practicalli-johnny]

Practicalli recommends using Neogit as the Neovim git client, especially when used to Magit style git client.

SPC g s keybinding is already taken by AstroNvim (to stage the current hunk). Rather than rebind this key mapping, the AstroNvim SPC g n t key mapping is used to launch Neogit in its own tab.

g t can be used to switch from Neogit to the code of a project, rather than using q to close Neogit each time. This means that the longer SPC g n t key sequence is only used once per session.

AstroNvim comes with SPC g g keybinding to launch lazygit (and external app run within a popup) which is convienient if you know how to use it.