Is your feature request related to a problem? Please describe.
I regularly use git reset --patch as the inverse of git add --patch and need to do this manually right now.

Describe the solution you'd like
It would be great if I could map <leader>hu to an unstage_hunk function instead of undo_stage_hunk which is limited to the last hunk that was staged.

Describe alternatives you've considered

• At first I expected that undo_stage_hunk to match git reset --patch, but of course this is not the case.
• I believe reset_hunk is effectively a --hard git reset, or git checkout --patch.
• reset_buffer_index unstages everything in the buffer which is usually not what I want

So I normally use :!git reset -p % which is more time consuming.

Additional context

• It's easy for me to understand the behavior of an unstage_hunk function because I frequently use git reset -p. However I expect it would be more clear to others if secondary signs as described in Sec #303 and Add signs for staged hunks #440. I expect that unstage_hunk would effectively operate on hunks indicated by secondary signs.

• Adding an optional range argument, similar to the range argument in stage_hunk(range) would also be handy. I've often found it difficult to edit patches with git reset --patch, so I'm not sure if this would be tricky to implement in practice.