Added a 'git diff --cached' alias -> 'gdc'

plugins/git/git.plugin.zsh

@@ -5,6 +5,8 @@ alias gst='git status'
 compdef _git gst=git-status
 alias gd='git diff'
 compdef _git gd=git-diff
+alias gdc='git diff --cached'
+compdef _git gdc=git-diff
 alias gl='git pull'
 compdef _git gl=git-pull
 alias gup='git pull --rebase'