builtin-commit: fix reflog message generation

Instead of strdup()ing, we can just reuse the buffer in which the
commit message is stored, and which is supposed to hold the reflog
message anyway.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin-commit.c

@@ -488,7 +488,7 @@ int cmd_commit(int argc, const char **argv, const char *prefix)
 	int header_len, parent_count = 0;
 	struct strbuf sb;
 	const char *index_file, *reflog_msg;
-	char *nl, *header_line;
+	char *nl;
 	unsigned char commit_sha1[20];
 	struct ref_lock *ref_lock;
 
@@ -585,12 +585,13 @@ int cmd_commit(int argc, const char **argv, const char *prefix)
 					   0);
 
 	nl = strchr(sb.buf + header_len, '\n');
-	header_line = xstrndup(sb.buf + header_len,
-			       nl - (sb.buf + header_len));
-	strbuf_release(&sb);
-	strbuf_addf(&sb, "%s: %s\n", reflog_msg, header_line);
-	strbuf_addch(&sb, '\0');
-	free(header_line);
+	if (nl)
+		strbuf_setlen(&sb, nl + 1 - sb.buf);
+	else
+		strbuf_addch(&sb, '\n');
+	strbuf_remove(&sb, 0, header_len);
+	strbuf_insert(&sb, 0, reflog_msg, strlen(reflog_msg));
+	strbuf_insert(&sb, strlen(reflog_msg), ": ", 2);
 
 	if (!ref_lock)
 		die("cannot lock HEAD ref");