completion: don't override given stash subcommand with -p

completion: don't override given stash subcommand with -p

df70b190 (completion: make stash -p and alias for stash push -p,
2018-04-20) wanted to make sure "git stash -p <TAB>" offers the same
completion as "git stash push -p <TAB>", but it did so by forcing the
$subcommand to be "push" whenever then "-p" option is found on the
command line.

This harms any subcommand that can take the "-p" option---even when the
subcommand is explicitly given, e.g. "git stash show -p", the code added
by the change would overwrite the $subcommand the user gave us.

Fix it by making sure that the defaulting to "push" happens only when
there is no $subcommand given yet.

Signed-off-by: Ville Skyttä <ville.skytta@iki.fi>
Signed-off-by: Junio C Hamano <gitster@pobox.com>