[Bug report] diff.noprefix config is ignored for interactive `add`

From: Nikita Bobko <nikitabobko@gmail.com>
Date: Apr 6, 2021, 10:15 UTC
Message-ID: <CAMJzOtyzu8y5mWdKXe3MPe8ZoJs8O=me8Xuu0t77YVdAMc7Tgg@mail.gmail.com>

Steps to reproduce:
1. Run `git -c diff.noprefix=true add -p`

Expected behavior: `a/` and `b/` prefixes are not shown during interactive `add` like this: ```

diff --git foo foo
index 257cc56..1715acd 100644
--- foo
+++ foo
@@ -1 +1 @@
-foo
+bar
(1/1) Stage this hunk [y,n,q,a,d,e,?]?
```

Actual behavior:
`a/` and `b/` prefixes are shown instead. Like this:
```
diff --git a/foo b/foo
index 257cc56..1715acd 100644
--- a/foo
+++ b/foo
@@ -1 +1 @@
-foo
+bar
(1/1) Stage this hunk [y,n,q,a,d,e,?]?
```

[System Info]
git version:
git version 2.30.1
cpu: x86_64
no commit associated with this build
sizeof-long: 8
sizeof-size_t: 8
shell-path: /bin/sh
uname: Linux 5.4.98-1-lts #1 SMP Sat, 13 Feb 2021 19:22:14 +0000 x86_64
compiler info: gnuc: 10.2
libc info: glibc: 2.33
$SHELL (typically, interactive shell): /bin/zsh

[Enabled Hooks]