The TortoiseGit client basically has this feature[1]. In the commit GUI, you can right click a file and select "Restore after commit", then you can edit the file to remove all the changes you don't want to commit (which means you can also test it, etc, and that your edits can be as granular as you want). Then, after you commit, it resets it to have all the changes you left out.
I like it a lot.
[1] https://stackoverflow.com/a/48788906