sha1-name: pass len argument to interpret_branch_name()

This is useful to make sure we don't step outside the boundaries of what
we are interpreting at the moment. For example while interpreting
foobar@{u}~1, the job of interpret_branch_name() ends right before ~1,
but there's no way to figure that out inside the function, unless the
len argument is passed.

So let's do that.

Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

refs.c

@@ -1951,7 +1951,7 @@ static int remove_empty_directories(const char *file)
 static char *substitute_branch_name(const char **string, int *len)
 {
 	struct strbuf buf = STRBUF_INIT;
-	int ret = interpret_branch_name(*string, &buf);
+	int ret = interpret_branch_name(*string, *len, &buf);
 
 	if (ret == *len) {
 		size_t size;