shorten_unambiguous_ref(): use NUM_REV_PARSE_RULES constant

The ref_rev_parse_rules[] array is terminated with a NULL entry, and we count it and store the result in the local nr_rules variable. But we don't need to do so; since the array is a constant, we can compute its size directly. The original code probably didn't do that because it was written as part of for-each-ref, and saw the array only as a pointer. It was migrated in 7c2b3029df (make get_short_ref a public function, 2009-04-07) and could have been updated then, but that subtlety was not noticed. We even have a constant that represents this value already, courtesy of 60650a48c0 (remote: make refspec follow the same disambiguation rule as local refs, 2018-08-01), though again, nobody noticed at the time that it could be used here, too. The current count-up isn't a big deal, as we need to preprocess that array anyway. But it will become more cumbersome as we refactor the shortening code. So let's get rid of it and just use the constant everywhere. Note that there are two things here that aren't just simple text replacements: 1. We also use nr_rules to see if a previous call has initialized the static pre-processing variables. We can just use the scanf_fmts pointer to do the same thing, as it is non-NULL only after we've done that initialization. 2. If nr_rules is zero after we've counted it up, we bail from the function. This code is unreachable, though, as the set of rules is hard-coded and non-empty. And that becomes even more apparent now that we are using the constant. So we can drop this conditional completely (and ironically, the code would have the same output if it _did_ trigger, as we'd simply skip the loop entirely and return the whole refname). Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>