expand --pretty=format color options

Currently, the only colors available to --pretty=format
users are red, green, and blue. Rather than expand it with a
few new colors, this patch makes the usual config color
syntax available, including more colors, backgrounds, and
attributes.

Because colors are no longer bounded to a single word (e.g.,
%Cred), this uses a more advanced syntax that features a
beginning and end delimiter (but the old syntax still
works). So you can now do:

  git log --pretty=tformat:'%C(yellow)%h%C(reset) %s'

to emulate --pretty=oneline, or even

  git log --pretty=tformat:'%C(cyan magenta bold)%s%C(reset)'

if you want to relive the awesomeness of 4-color CGA.

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

pretty.c

@@ -6,6 +6,7 @@
 #include "string-list.h"
 #include "mailmap.h"
 #include "log-tree.h"
+#include "color.h"
 
 static char *user_format;
 
@@ -554,6 +555,17 @@ static size_t format_commit_item(struct strbuf *sb, const char *placeholder,
 	/* these are independent of the commit */
 	switch (placeholder[0]) {
 	case 'C':
+		if (placeholder[1] == '(') {
+			const char *end = strchr(placeholder + 2, ')');
+			char color[COLOR_MAXLEN];
+			if (!end)
+				return 0;
+			color_parse_mem(placeholder + 2,
+					end - (placeholder + 2),
+					"--pretty format", color);
+			strbuf_addstr(sb, color);
+			return end - placeholder + 1;
+		}
 		if (!prefixcmp(placeholder + 1, "red")) {
 			strbuf_addstr(sb, "\033[31m");
 			return 4;