cmd/update: use short option in update.sh

Support the environment not supporting long option, such as Alpine Linux
with busybox.

Author: chirsz-ever

Library/Homebrew/cmd/update.sh

@@ -211,9 +211,8 @@ merge_or_rebase() {
   if [[ "${DIR}" == "${HOMEBREW_REPOSITORY}" && -n "${HOMEBREW_UPDATE_TO_TAG}" ]]
   then
     UPSTREAM_TAG="$(
-      git tag --list |
-        sort --field-separator=. --key=1,1nr -k 2,2nr -k 3,3nr |
-        grep --max-count=1 '^[0-9]*\.[0-9]*\.[0-9]*$'
+      git tag --list --sort=-version:refname |
+        grep -m 1 '^[0-9]*\.[0-9]*\.[0-9]*$'
     )"
   else
     UPSTREAM_TAG=""