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

tools/check_for_upgrade.sh

@@ -29,8 +29,10 @@ fi
 # Cancel upgrade if git is unavailable on the system
 whence git >/dev/null || return 0
 
-if [ -f ~/.zsh-update ]
+if mkdir "$ZSH/log/update.lock" 2>/dev/null
 then
+  if [ -f ~/.zsh-update ]
+  then
     . ~/.zsh-update
 
     if [[ -z "$LAST_EPOCH" ]]; then
@@ -53,7 +55,10 @@ then
         fi
       fi
     fi
-else
+  else
     # create the zsh file
     _update_zsh_update
+  fi
+
+  rm -r $ZSH/log/update.lock
 fi