parse_color: recognize "no$foo" to clear the $foo attribute

You can turn on ANSI text attributes like "reverse" by
putting "reverse" in your color spec. However, you cannot
ask to turn reverse off.

For common cases, this does not matter. You would turn on
"reverse" at the start of a colored section, and then clear
all attributes with a "reset". However, you may wish to turn
on some attributes, then selectively disable others. For
example:

  git log --format="%C(bold ul yellow)%h%C(noul) %s"

underlines just the hash, but without the need to re-specify
the rest of the attributes. This can also help third-party
programs, like contrib/diff-highlight, that want to turn
some attribute on/off without disrupting existing coloring.

Note that some attribute specifications are probably
nonsensical (e.g., "bold nobold"). We do not bother to flag
such constructs, and instead let the terminal sort it out.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/config.txt

@@ -824,7 +824,8 @@ accepted are `normal`, `black`, `red`, `green`, `yellow`, `blue`,
 `magenta`, `cyan` and `white`; the attributes are `bold`, `dim`, `ul`,
 `blink` and `reverse`.  The first color given is the foreground; the
 second is the background.  The position of the attribute, if any,
-doesn't matter.
+doesn't matter. Attributes may be turned off specifically by prefixing
+them with `no` (e.g., `noreverse`, `noul`, etc).
 +
 Colors (foreground and background) may also be given as numbers between
 0 and 255; these use ANSI 256-color mode (but note that not all

color.c

@@ -124,9 +124,11 @@ static int parse_color(struct color *out, const char *name, int len)
 
 static int parse_attr(const char *name, int len)
 {
-	static const int attr_values[] = { 1, 2, 4, 5, 7 };
+	static const int attr_values[] = { 1, 2, 4, 5, 7,
+					   22, 22, 24, 25, 27 };
 	static const char * const attr_names[] = {
-		"bold", "dim", "ul", "blink", "reverse"
+		"bold", "dim", "ul", "blink", "reverse",
+		"nobold", "nodim", "noul", "noblink", "noreverse"
 	};
 	int i;
 	for (i = 0; i < ARRAY_SIZE(attr_names); i++) {
@@ -238,7 +240,7 @@ int color_parse_mem(const char *value, int value_len, char *dst)
 			attr &= ~bit;
 			if (sep++)
 				*dst++ = ';';
-			*dst++ = '0' + i;
+			dst += sprintf(dst, "%d", i);
 		}
 		if (!color_empty(&fg)) {
 			if (sep++)

color.h

@@ -8,15 +8,15 @@ struct strbuf;
 /*
  * The maximum length of ANSI color sequence we would generate:
  * - leading ESC '['            2
- * - attr + ';'                 2 * 8 (e.g. "1;")
+ * - attr + ';'                 3 * 10 (e.g. "1;")
  * - fg color + ';'             17 (e.g. "38;2;255;255;255;")
  * - bg color + ';'             17 (e.g. "48;2;255;255;255;")
  * - terminating 'm' NUL        2
  *
  * The above overcounts attr (we only use 5 not 8) and one semicolon
  * but it is close enough.
  */
-#define COLOR_MAXLEN 56
+#define COLOR_MAXLEN 70
 
 /*
  * IMPORTANT: Due to the way these color codes are emulated on Windows,

t/t4026-color.sh

@@ -45,10 +45,21 @@ test_expect_success 'fg bg attr...' '
 	color "blue bold dim ul blink reverse" "[1;2;4;5;7;34m"
 '
 
+# note that nobold and nodim are the same code (22)
+test_expect_success 'attr negation' '
+	color "nobold nodim noul noblink noreverse" "[22;24;25;27m"
+'
+
 test_expect_success 'long color specification' '
 	color "254 255 bold dim ul blink reverse" "[1;2;4;5;7;38;5;254;48;5;255m"
 '
 
+test_expect_success 'absurdly long color specification' '
+	color \
+	  "#ffffff #ffffff bold nobold dim nodim ul noul blink noblink reverse noreverse" \
+	  "[1;2;4;5;7;22;24;25;27;38;2;255;255;255;48;2;255;255;255m"
+'
+
 test_expect_success '256 colors' '
 	color "254 bold 255" "[1;38;5;254;48;5;255m"
 '