Add git alias to clear entire stash (#5306)

Using the "gstc" alias runs "git stash clear"
that clears every stash index

plugins/git/git.plugin.zsh

@@ -217,6 +217,7 @@ alias gss='git status -s'
 alias gst='git status'
 alias gsta='git stash save'
 alias gstaa='git stash apply'
+alias gstc='git stash clear'
 alias gstd='git stash drop'
 alias gstl='git stash list'
 alias gstp='git stash pop'