rev-parse: exit with non-zero status if ref@{n} is not valid.

"The current behaviour of ref@{...} syntax parser is suboptimal: $ git rev-parse --verify jch@{99999} && echo true warning: Log for 'jch' only has 1368 entries. cfb88e9a8d4926b0011ae2dd67e1f57a98f4b768 true It even knows that it is running off the cut-off point; it should just cause the caller to notice that fact. I don't think changing it to error out should cause any harm to existing callers." With this change: $ git rev-parse --verify jch@{99999} || echo false fatal: Log for 'jch' only has 1368 entries. false $ git rev-parse jch@{99999} || echo false fatal: Log for 'jch' only has 1368 entries. false Signed-off-by: Jon Seymour <jon.seymour@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>