update-test: fix --to-tag failure on new tag.

Without this updating to a tag will always have the same start commit
and end commit as there's been no updates to `master` since the tag was
created.

Library/Homebrew/dev-cmd/update-test.rb

@@ -32,6 +32,8 @@ module Homebrew
       commit
     elsif date = ARGV.value("before")
       Utils.popen_read("git", "rev-list", "-n1", "--before=#{date}", "origin/master").chomp
+    elsif ARGV.include?("--to-tag")
+      Utils.popen_read("git", "tag", "--list", "--sort=-version:refname").lines[1].chomp
     else
       Utils.popen_read("git", "rev-parse", "origin/master").chomp
     end