add -p (built-in): do not color the progress indicator separately

add -p (built-in): do not color the progress indicator separately

The Perl version of this command colors the progress indicator and the
prompt message in one go.

Let's do the same in the built-in version so that the same upcoming test
(which will compare the output of `git add -p` against a known-good
version) will pass both for the Perl version as well as for the built-in
version.

Signed-off-by: Johannes Schindelin <redacted>
Signed-off-by: Junio C Hamano <redacted>