silence gcc array-bounds warning

silence gcc array-bounds warning

In shorten_unambiguous_ref, we build and cache a reverse-map of the
rev-parse rules like this:

  static char **scanf_fmts;
  static int nr_rules;
  if (!nr_rules) {
	  for (; ref_rev_parse_rules[nr_rules]; nr_rules++)
		  ... generate scanf_fmts ...
  }

where ref_rev_parse_rules is terminated with a NULL pointer.
Compiling with "gcc -O2 -Wall" does not cause any problems, but
compiling with "-O3 -Wall" generates:

  $ make CFLAGS='-O3 -Wall' refs.o
  refs.c: In function ‘shorten_unambiguous_ref’:
  refs.c:3379:29: warning: array subscript is above array bounds [-Warray-bounds]
     for (; ref_rev_parse_rules[nr_rules]; nr_rules++)

Curiously, we can silence this by explicitly nr_rules to 0
in the beginning of the loop, even though the compiler
should be able to tell that we follow this code path only
when nr_rules is already 0.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>