When using interactive add/commit (git add -p / git commit -p) it is supposed to prompt the user for input. Instead the diffs just get printed and the command never prompts for user input. Not sure where the problem lies but when I reverted back to dev-vcs/git-2.14.1 it worked as usual again.
Since git 2.14.2 isn't in portage and newer versions seem to work I'm marking this as resolved.