outdated: handle fully qualified tapped formulae.

Previously if I did `brew install boxen/brews/imagemagick` and then `brew
upgrade boxen/brews/imagemagick` and the version of `imagemagick` in
`boxen/brews` was older than the version in `Homebrew/homebrew` then it would
install the correct version from the `boxen/brews` tap and then try to
immediately upgrade it to the version from `Homebrew/homebrew`. I'd argue
fairly strongly that this behaviour is pretty unintuitive; when you fully
specify a formula from a tap then it should be prioritised by `brew upgrade`
and `brew outdated.

This commit makes `brew upgrade boxen/brews/imagemagick` only upgrade the
version of `imagemagick` if the version in the `boxen/brews` tap is newer.
Similarly `brew outdated imagemagick` and `brew outdated
boxen/brews/imagemagick` will show different results if the newer version
differs between `boxen/brews` and `Homebrew/homebrew`.

Closes Homebrew/homebrew#36699.

Signed-off-by: Mike McQuaid <mike@mikemcquaid.com>

Library/Homebrew/cmd/outdated.rb

@@ -17,9 +17,21 @@ module Homebrew
 
   def outdated_brews(formulae)
     formulae.map do |f|
-      versions = f.rack.subdirs.map { |d| Keg.new(d).version }.sort!
-        if versions.all? { |version| f.pkg_version > version }
-        yield f, versions if block_given?
+      all_versions = []
+      same_tap_versions = []
+      f.rack.subdirs.each do |dir|
+        keg = Keg.new dir
+        version = keg.version
+        all_versions << version
+
+        tap = Tab.for_keg(keg).tapped_from
+        if f.tap == tap || f.version == version
+          same_tap_versions << version
+        end
+      end
+
+      if same_tap_versions.all? { |version| f.pkg_version > version }
+        yield f, all_versions if block_given?
         f
       end
     end.compact