completion: stop showing 'save' for stash by default

completion: stop showing 'save' for stash by default

The 'save' subcommand in git stash has been deprecated in
fd2ebf14db ("stash: mark "git stash save" deprecated in the man page",
2017-10-22).

Stop showing it when the users enters 'git stash <tab>' or 'git stash
s<tab>'.  Keep showing it however when the user enters 'git stash sa<tab>'
or any more characters of the 'save' subcommand.  This is designed to
not encourage users to use 'git stash save', but still leaving the
completion option once it's clear that's what the user means.

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