Feature Request: Custom split location for `git add --patch`

L.S.

Please consider this feature request for interactive staging in patch mode, using [`git add --patch`][1].

## Problem

If a hunk cannot be split automatically, the `s` (split) option disappears from the list of options, and we get:

 > (1/6) Stage this hunk [y,n,q,a,d,j,J,g,/,e,?]? s
 > Sorry, cannot split this hunk

See for example these questions on StackOverflow: [2], [3]

I am aware that this problem can be solved by using the [`--edit` option][4] to edit the hunk, as explained in the docs under [editing patches][5].

However, if we have a large contiguous hunk, and we only want to split at one specific location, using `--edit` can be quite cumbersome, because (as far as I know) we need to edit all the lines that we do not want to stage.

## Feature request

Would it be possible to add a "custom split location" feature, allowing the user to specify where a contiguous hunk should be split?

Some options:

- Always show the `s` (split hunk) option, and let the user specify a 
line number if an automatic split cannot be made.

- Add a new [patch option][6], in addition to `s` (split hunk), to split 
any hunk at a custom location.

- Add a new operation for editing patches, for example "s" (in addition 
to "+", "-", and " "), indicating where to split (e.g. "split after this 
line"). This way the user only needs to modify a single line in edit mode.

I'm sure there are a lot of nasty details to consider, but I do think such a feature would be very convenient.

Thanks for your help,

Dennis

[1]: https://git-scm.com/docs/git-add#Documentation/git-add.txt---patch [2]: https://stackoverflow.com/q/6276752 [3]: https://stackoverflow.com/q/56892981 [4]: https://git-scm.com/docs/git-add#Documentation/git-add.txt---edit [5]: https://git-scm.com/docs/git-add#_editing_patches [6]: https://git-scm.com/docs/git-add#Documentation/git-add.txt-patch