color: support "default" to restore fg/bg color

The name "default" can now be used in foreground or background colors,
and means to use the terminal's default color, discarding any
explicitly-set color without affecting the other attributes. On many
modern terminals, this is *not* the same as specifying "white" or
"black".

Although attributes could previously be cleared like "no-bold", there
had not been a similar mechanism available for colors, other than a full
"reset", which cannot currently be combined with other settings.

Note that this is *not* the same as the existing name "normal", which is
a no-op placeholder to permit setting the background without changing
the foreground. (i.e. what is currently called "normal" might have been
more descriptively named "inherit", "none", "pass" or similar).

Signed-off-by: Robert Estelle <robertestelle@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/color.h b/color.h
index c20d324..27e8170 100644
--- a/color.h
+++ b/color.h
@@ -32,6 +32,7 @@ struct strbuf;
 #define GIT_COLOR_MAGENTA	"\033[35m"
 #define GIT_COLOR_CYAN		"\033[36m"
 #define GIT_COLOR_WHITE		"\033[37m"
+#define GIT_COLOR_DEFAULT	"\033[39m"
 #define GIT_COLOR_BOLD_BLACK	"\033[1;30m"
 #define GIT_COLOR_BOLD_RED	"\033[1;31m"
 #define GIT_COLOR_BOLD_GREEN	"\033[1;32m"
@@ -40,6 +41,7 @@ struct strbuf;
 #define GIT_COLOR_BOLD_MAGENTA	"\033[1;35m"
 #define GIT_COLOR_BOLD_CYAN	"\033[1;36m"
 #define GIT_COLOR_BOLD_WHITE	"\033[1;37m"
+#define GIT_COLOR_BOLD_DEFAULT	"\033[1;39m"
 #define GIT_COLOR_FAINT_BLACK	"\033[2;30m"
 #define GIT_COLOR_FAINT_RED	"\033[2;31m"
 #define GIT_COLOR_FAINT_GREEN	"\033[2;32m"
@@ -48,6 +50,7 @@ struct strbuf;
 #define GIT_COLOR_FAINT_MAGENTA	"\033[2;35m"
 #define GIT_COLOR_FAINT_CYAN	"\033[2;36m"
 #define GIT_COLOR_FAINT_WHITE	"\033[2;37m"
+#define GIT_COLOR_FAINT_DEFAULT	"\033[2;39m"
 #define GIT_COLOR_BG_BLACK	"\033[40m"
 #define GIT_COLOR_BG_RED	"\033[41m"
 #define GIT_COLOR_BG_GREEN	"\033[42m"
@@ -56,6 +59,7 @@ struct strbuf;
 #define GIT_COLOR_BG_MAGENTA	"\033[45m"
 #define GIT_COLOR_BG_CYAN	"\033[46m"
 #define GIT_COLOR_BG_WHITE	"\033[47m"
+#define GIT_COLOR_BG_DEFAULT	"\033[49m"
 #define GIT_COLOR_FAINT		"\033[2m"
 #define GIT_COLOR_FAINT_ITALIC	"\033[2;3m"
 #define GIT_COLOR_REVERSE	"\033[7m"