Use epochs in tabs

Library/Homebrew/formula.rb

@@ -436,6 +436,7 @@ class Formula
   def head_version_outdated?(version, options={})
     tab = Tab.for_keg(prefix(version))
 
+    return true if tab.version_scheme < version_scheme
     return true if stable && tab.stable_version && tab.stable_version < stable.version
     return true if devel && tab.devel_version && tab.devel_version < devel.version
 
@@ -1708,6 +1709,9 @@ class Formula
     # Used for creating new Homebrew versions schemes. For example, if we want
     # to change version scheme from one to another, then we may need to update
     # `version_scheme` of this {Formula} to be able to use new version scheme.
+    # E.g. to move from 20151020 scheme to 1.0.0 we need to increment
+    # `version_scheme`. Without this, the prior scheme will always equate to a
+    # higher version.
     # `0` if unset.
     #
     # <pre>version_scheme 1</pre>

Library/Homebrew/tab.rb

@@ -37,6 +37,7 @@ class Tab < OpenStruct
           "stable" => formula.stable ? formula.stable.version.to_s : nil,
           "devel" => formula.devel ? formula.devel.version.to_s : nil,
           "head" => formula.head ? formula.head.version.to_s : nil,
+          "version_scheme" => formula.version_scheme,
         }
       }
     }
@@ -78,6 +79,7 @@ class Tab < OpenStruct
         "stable" => nil,
         "devel" => nil,
         "head" => nil,
+        "version_scheme" => 0,
       }
     end
 
@@ -142,6 +144,7 @@ class Tab < OpenStruct
           "stable" => f.stable ? f.stable.version.to_s : nil,
           "devel" => f.devel ? f.devel.version.to_s : nil,
           "head" => f.head ? f.head.version.to_s : nil,
+          "version_scheme" => f.version_scheme,
         }
       }
     end
@@ -168,6 +171,7 @@ class Tab < OpenStruct
           "stable" => nil,
           "devel" => nil,
           "head" => nil,
+          "version_scheme" => 0,
         }
       }
     }
@@ -271,6 +275,10 @@ class Tab < OpenStruct
     Version.create(versions["head"]) if versions["head"]
   end
 
+  def version_scheme
+    versions["version_scheme"] || 0
+  end
+
   def source_modified_time
     Time.at(super)
   end