expand_user_path(): remove stale part of the comment

In 395de250d9 (Expand ~ and ~user in core.excludesfile,
commit.template, 2009-11-17), the `user_path()` function was refactored
into the `expand_user_path()`. During that refactoring, the `buf`
parameter was lost, but the code comment above said function still talks
about it. Let's remove that stale part of the comment.

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

path.c

@@ -719,9 +719,8 @@ static struct passwd *getpw_str(const char *username, size_t len)
 }
 
 /*
- * Return a string with ~ and ~user expanded via getpw*.  If buf != NULL,
- * then it is a newly allocated string. Returns NULL on getpw failure or
- * if path is NULL.
+ * Return a string with ~ and ~user expanded via getpw*. Returns NULL on getpw
+ * failure or if path is NULL.
  *
  * If real_home is true, strbuf_realpath($HOME) is used in the expansion.
  */