Regression in 'git add -p' with 'interactive.diffFilter' since builtin version is default

[Philippe Blain <levraiphilippeblain@gmail.com>]

Hi Dscho,

I found a regression in the builtin 'add -i'. I use diff-so-fancy [1]
as a pager and also use it via 'interactive.diffFilter'. With the 
builtin implementation, this errors with:

    error: could not parse colored hunk header '?[36m?[1m?[38;5;13m@ file:1 @?[1m?[0m'

With the Perl version, it works fine.

This has been reported in the 'diff-so-fancy' project here: [2].

Here is a reproducer:

~~~bash
#!/bin/bash

set -euEo pipefail

repo=repro
rm -rf $repo

TEST_AUTHOR_LOCALNAME=author
TEST_AUTHOR_DOMAIN=example.com
GIT_AUTHOR_EMAIL=${TEST_AUTHOR_LOCALNAME}@${TEST_AUTHOR_DOMAIN}
GIT_AUTHOR_NAME='A U Thor'
GIT_AUTHOR_DATE='1112354055 +0200'
TEST_COMMITTER_LOCALNAME=committer
TEST_COMMITTER_DOMAIN=example.com
GIT_COMMITTER_EMAIL=${TEST_COMMITTER_LOCALNAME}@${TEST_COMMITTER_DOMAIN}
GIT_COMMITTER_NAME='C O Mitter'
GIT_COMMITTER_DATE='1112354055 +0200'
export GIT_AUTHOR_EMAIL GIT_AUTHOR_NAME
export GIT_COMMITTER_EMAIL GIT_COMMITTER_NAME
export GIT_COMMITTER_DATE GIT_AUTHOR_DATE
export HOME=

git -c init.defaultBranch=unimportant init $repo
cd $repo
date>file
git add file
git commit -m file
git clone https://github.com/so-fancy/diff-so-fancy.git
date>>file
export PATH="diff-so-fancy:$PATH"
git -c interactive.diffFilter="diff-so-fancy --patch" -c add.interactive.useBuiltin=false add -p < <(echo y)
date>>file
git -c interactive.diffFilter="diff-so-fancy --patch" add -p
~~~

Cheers,

Philippe.

[1] https://github.com/so-fancy/diff-so-fancy
[2] https://github.com/so-fancy/diff-so-fancy/issues/437