merge-ort: implement format_commit()

This implementation is based on a mixture of print_commit() and
output_commit_title() from merge-recursive.c so that it can be used to
take over both functions.

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

merge-ort.c

@@ -328,7 +328,19 @@ static void format_commit(struct strbuf *sb,
 			  int indent,
 			  struct commit *commit)
 {
-	die("Not yet implemented.");
+	struct merge_remote_desc *desc;
+	struct pretty_print_context ctx = {0};
+	ctx.abbrev = DEFAULT_ABBREV;
+
+	strbuf_addchars(sb, ' ', indent);
+	desc = merge_remote_util(commit);
+	if (desc) {
+		strbuf_addf(sb, "virtual %s\n", desc->name);
+		return;
+	}
+
+	format_commit_message(commit, "%h %s", sb, &ctx);
+	strbuf_addch(sb, '\n');
 }
 
 __attribute__((format (printf, 4, 5)))