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>

contrib/completion/git-completion.bash

@@ -2776,6 +2776,9 @@ _git_stash ()
 	local save_opts='--all --keep-index --no-keep-index --quiet --patch --include-untracked'
 	local subcommands='push list show apply clear drop pop create branch'
 	local subcommand="$(__git_find_on_cmdline "$subcommands save")"
+	if [ -n "$(__git_find_on_cmdline "-p")" ]; then
+		subcommand="push"
+	fi
 	if [ -z "$subcommand" ]; then
 		case "$cur" in
 		--*)