stash: declare ref_stash as an array

Save sizeof(const char *) bytes by declaring ref_stash as an array
instead of having a redundant pointer to an array.

Signed-off-by: Denton Liu <liu.denton@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/stash.c

@@ -87,7 +87,7 @@ static const char * const git_stash_save_usage[] = {
 	NULL
 };
 
-static const char *ref_stash = "refs/stash";
+static const char ref_stash[] = "refs/stash";
 static struct strbuf stash_index_path = STRBUF_INIT;
 
 /*