Implement a locking mechanism to avoid multiple update prompts (fixes #3766)

tools/check_for_upgrade.sh

@@ -29,6 +29,8 @@ fi
 # Cancel upgrade if git is unavailable on the system
 whence git >/dev/null || return 0
 
+if mkdir "$ZSH/log/update.lock" 2>/dev/null
+then
   if [ -f ~/.zsh-update ]
   then
     . ~/.zsh-update
@@ -57,3 +59,6 @@ else
     # create the zsh file
     _update_zsh_update
   fi
+
+  rm -r $ZSH/log/update.lock
+fi