updater: add --autostash to git pull (#7172)

If I have custom configs (like theme customizations) I have to stash my changes and get them back after the update. 

By adding the --autostash on upgrade.sh, if I have any changes not commited they'll be reapplied after the upgrade, allowing me to have temporary customizations without any harm to the upgrade process.

tools/upgrade.sh

@@ -38,7 +38,7 @@ if [ -n "$remote" ]; then
 fi
 
 printf "${BLUE}%s${NORMAL}\n" "Updating Oh My Zsh"
-if git pull --rebase --stat origin master
+if git pull --rebase --autostash --stat origin master
 then
   printf '%s' "$GREEN"
   printf '%s\n' '         __                                     __   '