Q:

Stage diff hunk into git from ediff? 

How do you stage a hunk into git from ediff? 
I'd like to be able to view a side-by-side diff and stage hunks into the "HEAD" side of the diff, but that buffer (i.e., spacemacs.~master~) is read-only.  Ideally, I could also edit that staged file (to achieve the precision I'd get from editing a hunk from git command-line). 
It looks like I can invoke magit status (M-x magit-status), expand a diff (TAB), and use s to stage a diff hunk, but I'd like to do this on a file-by-file basis and from a side-by-side diff. 
I'm hoping to replicate the behavior of vim-fugitive. 

A:

Move point to the file you want to partially stage/unstage using Ediff and press e (magit-ediff-dwim).  Alternatively you can also leave point where it is and press E s (magit-ediff-stage) but then you will be asked about the file in the minibuffer - so moving to the file first might be more convenient. 
In the resulting Ediff session use c b to stage and a b to unstage.  Then finish the session using q. You'll be asked whether you really want to finish the session.  Then you'll also be asked whether you want to update the index with the contents of the buffer representing the index in Ediff.  You do - the Ediff commands used previously only modified the buffer without modifying the actual index.