completion: make stash -p and alias for stash push -p

completion: make stash -p and alias for stash push -p

We define 'git stash -p' as an alias for 'git stash push -p' in the
manpage.  Do the same in the completion script, so all options that
can be given to 'git stash push' are being completed when the user is
using 'git stash -p --<tab>'.  Currently the only additional option
the user will get is '--message', but there may be more in the future.

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