Fix conflict with common zsh alias

oh-my-zsh common-aliases plugin defines `alias -g T='| tail'`, which causes an
error with the line `local T dir f`.

Making these lines more explicit resolves this issue, while also being clearer
that the intent is to clear out the local variables.

Bug: 169191000
Fix: 169191000
Test: source build/envsetup.sh; lunch <target>; m
Change-Id: Ief27aa3d9f541d0c39c6c11d7d771818d6493d31

envsetup.sh

@@ -356,7 +356,7 @@ function should_add_completion() {
 
 function addcompletions()
 {
-    local T dir f
+    local f=
 
     # Keep us from trying to run in something that's neither bash nor zsh.
     if [ -z "$BASH_VERSION" -a -z "$ZSH_VERSION" ]; then