stash: convert show to builtin

stash: convert show to builtin

Add stash show to the helper and delete the show_stash, have_stash,
assert_stash_like, is_stash_like and parse_flags_and_rev functions
from the shell script now that they are no longer needed.

In shell version, although `git stash show` accepts `--index` and
`--quiet` options, it ignores them. In C, both options are passed
further to `git diff`.

Signed-off-by: Paul-Sebastian Ungureanu <ungureanupaulsebastian@gmail.com>
Signed-off-by: Thomas Gummerer <t.gummerer@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>