formula: use names for formula comparison

* Causes a bug in Formula#installed_alias_target_changed? when
  Formula#superseds_an_installed_formula? returns true because
  Formula#old_installed_formulae includes f for some Formula f.

* Causes a bug when foo@2.4 with alias foo has HEAD or devel version and
  we try to `brew upgrade foo --devel|--HEAD` from stable. The upgrade fails
  while since we installing formula to the same prefix it's alredy installed.
  The reason for that is that we use
  `formula_to_install = outdated.map(&:latest_formula)` in cmd/upgrade
  before calling upgrade_formula on foo.

  ```ruby
  def latest_formula
  installed_alias_target_changed? ? current_installed_alias_target : self
  end
  ```

  Formula#installed_alias_target_changed? compares formulae using
  Formula#==, which is wrong for this case, thus Formula#latest_formula doesn't
  return self and returns Formula#current_installed_alias_target with spec
  foo was initially installed instead of devel or HEAD, causing the error.

Library/Homebrew/formula.rb

@@ -1167,7 +1167,7 @@ class Formula
   # Returns false if the formula wasn't installed with an alias.
   def installed_alias_target_changed?
     target = current_installed_alias_target
-    target && target != self
+    target && target.name != name
   end
 
   # Is this formula the target of an alias used to install an old formula?
@@ -1192,7 +1192,7 @@ class Formula
     # 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]
+    self.class.installed_with_alias_path(alias_path).reject { |f| f.name == name }
   end
 
   # @private