symbolit-ref: fix resolve_ref conversion.

An earlier conversion accidentally hardcoded "HEAD" to be passed to
resolve_ref(), thereby causing git-symbolic-ref command to always
report where the HEAD points at, ignoring the command line parameter.
Signed-off-by: NJunio C Hamano <junkio@cox.net>

builtin-symbolic-ref.c

@@ -7,7 +7,7 @@ static const char git_symbolic_ref_usage[] =
 static void check_symref(const char *HEAD)
 {
 	unsigned char sha1[20];
-	const char *refs_heads_master = resolve_ref("HEAD", sha1, 0);
+	const char *refs_heads_master = resolve_ref(HEAD, sha1, 0);
 
 	if (!refs_heads_master)
 		die("No such ref: %s", HEAD);