add '%d' pretty format specifier to show decoration

Add a new format placeholder, %d, which expands to a ref name decoration
(think git log --decorate).  It expands to an empty string if the commit
has no decoration, or otherwise to a comma (and space) separated list of
decorations, surrounded by parentheses and a leading space.

Michael Dressel implemented an initial version and chose the letter d,
Junio suggested to add a leading space and parentheses.

Signed-off-by: Rene Scharfe <rene.scharfe@lsrfire.ath.cx>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/pretty-formats.txt

@@ -116,6 +116,7 @@ The placeholders are:
 - '%cr': committer date, relative
 - '%ct': committer date, UNIX timestamp
 - '%ci': committer date, ISO 8601 format
+- '%d': ref names, like the --decorate option of linkgit:git-log[1]
 - '%e': encoding
 - '%s': subject
 - '%b': body

pretty.c

@@ -5,6 +5,7 @@
 #include "revision.h"
 #include "string-list.h"
 #include "mailmap.h"
+#include "log-tree.h"
 
 static char *user_format;
 
@@ -481,6 +482,23 @@ static void parse_commit_header(struct format_commit_context *context)
 	context->commit_header_parsed = 1;
 }
 
+static void format_decoration(struct strbuf *sb, const struct commit *commit)
+{
+	struct name_decoration *d;
+	const char *prefix = " (";
+
+	load_ref_decorations();
+	d = lookup_decoration(&name_decoration, &commit->object);
+	while (d) {
+		strbuf_addstr(sb, prefix);
+		prefix = ", ";
+		strbuf_addstr(sb, d->name);
+		d = d->next;
+	}
+	if (prefix[0] == ',')
+		strbuf_addch(sb, ')');
+}
+
 static size_t format_commit_item(struct strbuf *sb, const char *placeholder,
                                void *context)
 {
@@ -573,6 +591,9 @@ static size_t format_commit_item(struct strbuf *sb, const char *placeholder,
 		                 ? '<'
 		                 : '>');
 		return 1;
+	case 'd':
+		format_decoration(sb, commit);
+		return 1;
 	}
 
 	/* For the rest we have to parse the commit header. */