color: add the meta color GIT_COLOR_REVERSE

This "color" simply reverts background and foreground. It will be used
in the upcoming "dual color" mode of `git range-diff`, where we will
reverse colors for the -/+ markers and the fragment headers of the
"outer" diff.
Signed-off-by: NJohannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

color.h

@@ -44,6 +44,7 @@ struct strbuf;
 #define GIT_COLOR_BG_CYAN	"\033[46m"
 #define GIT_COLOR_FAINT		"\033[2m"
 #define GIT_COLOR_FAINT_ITALIC	"\033[2;3m"
+#define GIT_COLOR_REVERSE	"\033[7m"
 
 /* A special value meaning "no color selected" */
 #define GIT_COLOR_NIL "NIL"