Replace dirname with zsh expansion modifier

- [x] No leading whitespace
- [x] No external command
- [x] More cryptic

git-open.plugin.zsh

@@ -3,4 +3,4 @@
 # Make git-open easy to install and keep up to date if you're using a
 # ZSH framework like Zgen or Antigen
 
-  export PATH=$(dirname $0):${PATH}
+export PATH=${0:A:h}:${PATH}