bash: add new 'git stash' subcommands

Namely 'save', 'drop', 'pop' and 'create'

Signed-off-by: SZEDER Gábor <szeder@ira.uka.de>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>

contrib/completion/git-completion.bash

@@ -1119,7 +1119,7 @@ _git_show ()
 
 _git_stash ()
 {
-	local subcommands='list show apply clear'
+	local subcommands='save list show apply clear drop pop create'
 	if [ -z "$(__git_find_subcommand "$subcommands")" ]; then
 		__gitcomp "$subcommands"
 	fi