detached-stash: simplify stash_drop

detached-stash: simplify stash_drop

Previously, git stash drop would fail noisily while executing git reflog
delete if the specified revision was not a stash reference.

Now, git stash drop fails with an error message which more precisely
indicates the reason for failure.

Furthermore, git stash drop will now fail with a non-zero status code
if stash@{n} specifies a stash log entry that does not actually exist.

This change in behaviour is achieved by delegating argument parsing
to the common parse_flags_and_rev() function (via a call to
assert_stash_ref).

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