Add a keybinding for 'git add --intent-to-add' ('git add -D')

Is your feature request related to a problem? Please describe.

Sometimes I want to see a diff in Neogit for a file that hasn't been added yet. The only way to do it inside Neogit is by staging it (pressing s).

Describe the solution you'd like

The git add command supports the flag --intent-to-add (docs here). Neogit could support it with a top-level keybinding for the NeogitStatus buffer, for example i.

Pressing i would run git add --intent-to-add <file-under-cursor> and result in this:

Note that the file is in the Unstaged area.

Describe alternatives you've considered

Run git add --intent-to-add <file-under-cursor> in the CLI. Defeats the purpose of Neogit.

Start at the NeogitStatus buffer's top-level keybinding definitions and trace how the file under the cursor is passed to Git commands. Add the requested intent-to-add action and verify that pressing the binding runs git add --intent-to-add <file> while leaving the file in the Unstaged area.