pretty.c: delimit "%(trailers)" arguments with ","

In preparation for adding consistent "%(trailers)" atom options to
`git-for-each-ref(1)`'s "--format" argument, change "%(trailers)" in
pretty.c to separate sub-arguments with a ",", instead of a ":".

Multiple sub-arguments are given either as "%(trailers:unfold,only)" or
"%(trailers:only,unfold)".

This change disambiguates between "top-level" arguments, and arguments
given to the trailers atom itself. It is consistent with the behavior of
"%(upstream)" and "%(push)" atoms.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

pretty.c

@@ -1056,6 +1056,24 @@ static size_t parse_padding_placeholder(struct strbuf *sb,
 	return 0;
 }
 
+static int match_placeholder_arg(const char *to_parse, const char *candidate,
+				 const char **end)
+{
+	const char *p;
+
+	if (!(skip_prefix(to_parse, candidate, &p)))
+		return 0;
+	if (*p == ',') {
+		*end = p + 1;
+		return 1;
+	}
+	if (*p == ')') {
+		*end = p;
+		return 1;
+	}
+	return 0;
+}
+
 static size_t format_commit_one(struct strbuf *sb, /* in UTF-8 */
 				const char *placeholder,
 				void *context)
@@ -1285,11 +1303,16 @@ static size_t format_commit_one(struct strbuf *sb, /* in UTF-8 */
 
 	if (skip_prefix(placeholder, "(trailers", &arg)) {
 		struct process_trailer_options opts = PROCESS_TRAILER_OPTIONS_INIT;
-		while (*arg == ':') {
-			if (skip_prefix(arg, ":only", &arg))
-				opts.only_trailers = 1;
-			else if (skip_prefix(arg, ":unfold", &arg))
-				opts.unfold = 1;
+		if (*arg == ':') {
+			arg++;
+			for (;;) {
+				if (match_placeholder_arg(arg, "only", &arg))
+					opts.only_trailers = 1;
+				else if (match_placeholder_arg(arg, "unfold", &arg))
+					opts.unfold = 1;
+				else
+					break;
+			}
 		}
 		if (*arg == ')') {
 			format_trailers_from_commit(sb, msg + c->subject_off, &opts);

t/t4205-log-pretty-formats.sh

@@ -588,8 +588,8 @@ test_expect_success '%(trailers:unfold) unfolds trailers' '
 '
 
 test_expect_success ':only and :unfold work together' '
-	git log --no-walk --pretty="%(trailers:only:unfold)" >actual &&
-	git log --no-walk --pretty="%(trailers:unfold:only)" >reverse &&
+	git log --no-walk --pretty="%(trailers:only,unfold)" >actual &&
+	git log --no-walk --pretty="%(trailers:unfold,only)" >reverse &&
 	test_cmp actual reverse &&
 	{
 		grep -v patch.description <trailers | unfold &&