add git difftool alias (gdt)

plugins/git/git.plugin.zsh

@@ -18,6 +18,7 @@ compdef _git gp=git-push
 alias gd='git diff'
 gdv() { git diff -w "$@" | view - }
 compdef _git gdv=git-diff
+alias gdt='git difftool'
 alias gc='git commit -v'
 compdef _git gc=git-commit
 alias gc!='git commit -v --amend'