1. Remove lines beginning with + to keep them from being added
2. Replace the beginning - with <space> for lines to keep them from being removed
But you can go further with this by adding more lines beginning with + to add lines that weren't in the original hunk, or add a - prefix to a context line (beginning with a <space>) to remove it. You can even modify the content of lines prefixed with + (i.e., to fix a syntax error you noticed) before staging them.
The other thing to note is the surrounding context lines for a given hunk. Ideally, you have at least 3 lines before and 3 lines after the hunk to ensure that when it's applied, it changes the file in the correct location.
In order to actually stage a hunk, you need to add the necessary header lines above the line that begins with @@. These lines are the ones that begin with diff, index, ---, and +++. For the first, third and fourth lines, the path and file name need to match the file you intend to apply the patch to.
When you're ready to stage your edited hunk, you just visually highlight it in vim (shift-v) and then run
:'<,'> !git apply --cached --recount
git status -v
git diff --cached
git show :0:path/to/file
I can then run git status -v to show what's currently staged, write the commit message and filter it through the git commit command.
> there’s no way you can review (and pick or discard) your 100-lines 5-files changes effectively on the command line.
It's easy enough to find the approriate hunk and edit the diff to stage what's needed. I may go through the process a few times to create a single commit depending on how many hunks I have to deal with and how many times I need to split them though.
In fact, my normal workflow is to make the necessary changes for a particular feature I'm working on, take the diff against the base branch and then stage parts of that diff to create a series of sensible commits for the branch.