sha1_file: improve sha1_file_name() perfs

As sha1_file_name() could be performance sensitive, let's
make it faster by using strbuf_addstr() and strbuf_addc()
instead of strbuf_addf().

Helped-by: Derrick Stolee <stolee@gmail.com>
Helped-by: Jeff Hostetler <git@jeffhostetler.com>
Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

sha1_file.c

@@ -253,8 +253,8 @@ static void fill_sha1_path(struct strbuf *buf, const unsigned char *sha1)
 
 void sha1_file_name(struct strbuf *buf, const unsigned char *sha1)
 {
-	strbuf_addf(buf, "%s/", get_object_directory());
-
+	strbuf_addstr(buf, get_object_directory());
+	strbuf_addch(buf, '/');
 	fill_sha1_path(buf, sha1);
 }