color: allow multiple attributes

In configuration files (and "git config --color" command line), we
supported one and only one attribute after foreground and background
color.  Accept combinations of attributes, e.g.

    [diff.color]
            old = red reverse bold

Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/color.h b/color.h
index 5019df8..e2988f4 100644
--- a/color.h
+++ b/color.h
@@ -1,8 +1,20 @@
 #ifndef COLOR_H
 #define COLOR_H
 
-/* "\033[1;38;5;2xx;48;5;2xxm\0" is 23 bytes */
-#define COLOR_MAXLEN 24
+/*  2 + (2 * num_attrs) + 8 + 1 + 8 + 'm' + NUL */
+/* "\033[1;2;4;5;7;38;5;2xx;48;5;2xxm\0" */
+/*
+ * The maximum length of ANSI color sequence we would generate:
+ * - leading ESC '['            2
+ * - attr + ';'                 2 * 8 (e.g. "1;")
+ * - fg color + ';'             9 (e.g. "38;5;2xx;")
+ * - fg color + ';'             9 (e.g. "48;5;2xx;")
+ * - 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 40
 
 /*
  * This variable stores the value of color.ui