Add git alias for staging tracked files (#5178)

Although `gaa` (git add --all) is cool, it stages every file, tracked or
not, if it is not on .gitignore.

Sometimes we want to just stage everything we are working on, that is
already tracked. For that reason, 'gau' can save us some time.

plugins/git/git.plugin.zsh

@@ -43,6 +43,7 @@ alias g='git'
 alias ga='git add'
 alias gaa='git add --all'
 alias gapa='git add --patch'
+alias gau='git add --update'
 
 alias gb='git branch'
 alias gba='git branch -a'