git-gui: apply color information from git diff output

git-gui: apply color information from git diff output

This patch extracts the ANSI color sequences from git diff output and
applies these to the diff view window. This ensures that the gui view
makes use of the current git configuration for whitespace display.

ANSI codes may include attributes, foreground and background in a single
sequence. Handle this and support bold and reverse attributes. Ignore
all other attributes.

Suggested-by: Tor Arvid Lund <torarvid@gmail.com>
Suggested-by: Junio C Hamano <gitster@pobox.com>
Tested-by: Tor Arvid Lund <torarvid@gmail.com>
Signed-off-by: Pat Thoyts <patthoyts@users.sourceforge.net>