completion: avoid aliased command lookup error in nounset mode

Aliased command lookup accesses the `list` variable before it has been
set, causing an error in "nounset" mode. Initialize to an empty string
to avoid that.

    $ git nonexistent-command <Tab>bash: list: unbound variable

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

contrib/completion/git-completion.bash

@@ -1129,7 +1129,7 @@ __git_pretty_aliases ()
 # __git_aliased_command requires 1 argument
 __git_aliased_command ()
 {
-	local cur=$1 last list word cmdline
+	local cur=$1 last list= word cmdline
 
 	while [[ -n "$cur" ]]; do
 		if [[ "$list" == *" $cur "* ]]; then