t4052: test for diffstat width when prefix contains ANSI escape codes

t4052: test for diffstat width when prefix contains ANSI escape codes

Add test checking the calculation of the diffstat display width when the
`line_prefix`, which is text that goes before the diffstat, contains
ANSI escape codes.

This situation happens, for example, when `git log --stat --graph` is
executed:
* `--stat` will create a diffstat for each commit
* `--graph` will stuff `line_prefix` with the graph portion of the log,
  which contains ANSI escape codes to color the text

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