git plugin: fix error msg in gwip alias

because git rm will fail when no files were deleted

plugins/git/git.plugin.zsh

@@ -204,4 +204,4 @@ alias gupv='git pull --rebase -v'
 alias gvt='git verify-tag'
 
 alias gwch='git whatchanged -p --abbrev-commit --pretty=medium'
-alias gwip='git add -A; git rm $(git ls-files --deleted); git commit -m "--wip--"'
+alias gwip='git add -A; git rm $(git ls-files --deleted) 2> /dev/null; git commit -m "--wip--"'