stash: convert drop and clear to builtin

stash: convert drop and clear to builtin

Add the drop and clear commands to the builtin helper. These two
are each simple, but are being added together as they are quite
related.

We have to unfortunately keep the drop and clear functions in the
shell script as functions are called with parameters internally
that are not valid when the commands are called externally. Once
pop is converted they can both be removed.

Signed-off-by: Joel Teichroeb <joel@teichroeb.net>
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>