diff: handle ANSI escape codes in prefix when calculating diffstat width

diff: handle ANSI escape codes in prefix when calculating diffstat width

The diffstat width is calculated by taking the terminal width and
incorrectly subtracting the `strlen()` of `line_prefix`, instead of the
actual display width of `line_prefix`, which may contain ANSI escape
codes (e.g., ANSI-colored strings in `log --graph --stat`).

Utilize the display width instead, obtained via `utf8_strnwidth()` with
the flag `skip_ansi`.

Signed-off-by: LorenzoPegorari <lorenzo.pegorari2002@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>