Formula#old_installed_formula?: fix some bugs

Previously, this method would:

- Include the current formula
- Include the current target of the installed alias, which by definition
  is not "old"

This commit fixes both of these issues.
Only formulae that are the current target of the alias they were installed
with are now considered to have old installed formulae.

Library/Homebrew/formula.rb

@@ -1186,7 +1186,11 @@ class Formula
   end
 
   def old_installed_formulae
-    alias_path ? self.class.installed_with_alias_path(alias_path) : []
+    # If this formula isn't the current target of the alias,
+    # it doesn't make sense to say that other formulae are older versions of it
+    # because we don't know which came first.
+    return [] if alias_path.nil? || installed_alias_target_changed?
+    self.class.installed_with_alias_path(alias_path) - [self]
   end
 
   # @private