stash: eliminate crude option parsing

stash: eliminate crude option parsing

Eliminate crude option parsing and rely on real parsing instead, because
1) Crude parsing is crude, for example it's not capable of
   handling things like `git stash -m Message`
2) Adding options in two places is inconvenient and prone to bugs

As a side result, the case of `git stash -m Message` gets fixed.
Also give a good error message instead of just throwing usage at user.

----

Some review of what's been happening to this code:

Before [1], `git-stash.sh` only verified that all args begin with `-` :

	# The default command is "push" if nothing but options are given
	seen_non_option=
	for opt
	do
		case "$opt" in
		--) break ;;
		-*) ;;
		*) seen_non_option=t; break ;;
		esac
	done

Later, [1] introduced the duplicate code I'm now removing, also making
the previous test more strict by white-listing options.

----

[1] Commit 40af1468 ("stash: convert `stash--helper.c` into `stash.c`" 2019-02-26)

Signed-off-by: Alexandr Miloslavskiy <alexandr.miloslavskiy@syntevo.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>