Re: [PATCH 3/3] --format=pretty: avoid calculating expensive expansions twice

+static int add_again(struct strbuf *sb, struct chunk *chunk)
+{
+	if (chunk->len) {
+		strbuf_adddup(sb, chunk->off, chunk->len);
+		return 1;
+	}
+
+	/*
+	 * We haven't seen this chunk before.  Our caller is surely
+	 * going to add it the hard way now.  Remember the most likely
+	 * start of the to-be-added chunk: the current end of the
+	 * struct strbuf.
+	 */
+	chunk->off = sb->len;
+	return 0;
+}
+
 static void parse_commit_header(struct format_commit_context *context)
 {
 	const char *msg = context->commit->buffer;

@@ -447,15 +469,21 @@ static void format_commit_item(struct strbuf *sb, const char *placeholder,
 		strbuf_addstr(sb, sha1_to_hex(commit->object.sha1));
 		return;
 	case 'h':		/* abbreviated commit hash */
+		if (add_again(sb, &c->abbrev_commit_hash))
+			return;
 		strbuf_addstr(sb, find_unique_abbrev(commit->object.sha1,
 		                                     DEFAULT_ABBREV));
+		c->abbrev_commit_hash.len = sb->len - c->abbrev_commit_hash.off;
 		return;