stash: fix accidental apply of non-existent stashes

stash: fix accidental apply of non-existent stashes

Once upon a time, "git rev-parse ref@{9999999}" did not
generate an error. Therefore when we got an invalid stash
reference in "stash apply", we could end up not noticing
until quite late.  Commit b0f0ecd (detached-stash: work
around git rev-parse failure to detect bad log refs,
2010-08-21) handled this by checking for the "Log for stash
has only %d entries" warning on stderr when we validated the
ref.

A few days later, e6eedc3 (rev-parse: exit with non-zero
status if ref@{n} is not valid., 2010-08-24) fixed the
original issue. That made the extra stderr test superfluous,
but also introduced a new bug. Now the early call to:

  git rev-parse --symbolic "$@"

fails, but we don't notice the exit code. Worse, its empty
output means we think the user didn't provide us a ref, and
we try to apply stash@{0}.

This patch checks the rev-parse exit code and fails early in
the revision parsing process. We can also get rid of the
stderr test; as a bonus, this means that "stash apply" can
now run under GIT_TRACE=1 properly.

Signed-off-by: Jeff King <peff@peff.net>
Acked-by: Jon Seymour <jon.seymour@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>