update-test: tweak master not updated check.

Loosen this a bit; we don't necessarily expect the end commit is the one
we're looking for, just that it has changed from the start commit (i.e.
some sort of update has occurred).

Addresses some false negatives on `master` branch merges that weren't
present on the PR commits.

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

@@ -49,7 +49,7 @@ module Homebrew
       oh1 "Running brew update..."
       safe_system "brew", "update", "--verbose"
       actual_end_sha1 = Utils.popen_read("git", "rev-parse", "master").chomp
-      if actual_end_sha1 != end_sha1
+      if start_sha1 != end_sha1 && start_sha1 == actual_end_sha1
         raise <<-EOS.undent
           brew update didn't update master!
           Start commit:        #{start_sha1}