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>