Discussion for interactive --patch commands to get --unified support

Hi everyone!

I am a big fan of `git diff` with a custom `diff.context` setting to increase the context lines (which also extends to other commands like `git show` which I really like). As well as this, I am also a frequent user of `git add --patch` (and also the other interactive `--patch` variants such as reset).

As I've grown to use and appreciate these features even more, I have noticed and been bothered that `git add --patch` doesn't have a (easy) way of configuring how many context lines you see. There is a stackoverflow post (https://stackoverflow.com/questions/6711670/git-show-more-context-when-using-git-add-i-or-git-add-e) which mentions you can do `GIT_DIFF_OPTS=-u<number> git add -p` which does work however isn't very user friendly or convenient.

It would be great if it was possible to start a discussion how this
could be made better (and I would be happy to submit a patch if all is
good). Whilst brainstorming briefly, here are a few options I have
thought of that could solve this pain point, some not mutually
exclusive:
- `-U<number>/--unified=<number>` command line options to the
interactive patch commands (all builtins who call `run_add_p`)
- make `diff.context` setting extend to the interactive patch commands
(not sure how a change like this would be welcomed considering it
could change users command outputs seemingly out of nowhere)
- add an `interactive.context` setting that would work like the
existing `diff.context` setting but apply only to the interactive
patch commands

Is this feature something people would welcome, and what are your thoughts?

Thanks!