Marking (flagging) files for later action (stage/unstage/discard)

**Is your feature request related to a problem? Please describe.**
I'm always frustrated when I have to stage/unstage/discard multiple files in "files" menu. Git is slow on Windows in a huge repo and if I wan to do those action only on some files it takes ages.

**Describe the solution you'd like**
Allow marking a file for later action dired style. You would simply select multiple files with a keybinding (there should be visual feedback of a file being marked) and then press another keybinding to do action (d would discard all marked files)

**Describe alternatives you've considered**
There's no alternative that I know of that would achieve this. Main point is performance and this adds no new functionality in terms of what you can do with git.

**Additional context**
As I've already said I'm on Windows and in huge repo with thousands of commits/files staging/unstaging/discrading files can take up quite some time. This change would speed up the process and might unlock other workflows that are not apparent with my use case.