bug#52349: 29.0.50; vc-git and diff-mode: stage hunks

[Juri Linkov]

>> It's probably something diff-hl does as well by now (I have not checked).
>
> It does, since recently.
>
> But Juri's approach seems like it will fit VC's design better, given that
> we don't have a semantic way to "show all stages hunks" or commit them only
> (or instruct Emacs to do that).

The problem is that 'git apply --cached' doesn't perform the merge
with other changes in the same file, whereas 'git stash pop'
merges committed changes with uncommitted changes.