stash: convert `stash--helper.c` into `stash.c`

stash: convert `stash--helper.c` into `stash.c`

The old shell script `git-stash.sh`  was removed and replaced
entirely by `builtin/stash.c`. In order to do that, `create` and
`push` were adapted to work without `stash.sh`. For example, before
this commit, `git stash create` called `git stash--helper create
--message "$*"`. If it called `git stash--helper create "$@"`, then
some of these changes wouldn't have been necessary.

This commit also removes the word `helper` since now stash is
called directly and not by a shell script.

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>