Ignore tracked file/Ability to add options to built in dialogs

[Mach1212]

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

Adjacent to #3380

Describe the solution you'd like

I want the ability to add an option to the "Ignore or exclude file" dialog that pops up when I press "i" in the files tab.
Or I want git update-index --assume-unchanged to be added as a built in command.

Describe alternatives you've considered

I've read the custom commands section and couldn't find it. Then Grok told me that builtin dialogs are not customizable? Seems like an oversight...

Additional context

[stefanhaller]

For a discussion about custom commands to use for ignoring local files, see #2914 (comment).

(I think you want --skip-worktree instead of --assume-unchanged, see here.)