Reset repo_var in clear_cache.

Library/Homebrew/compat/tap.rb

@@ -15,10 +15,9 @@ class Tap
       old_path = path
       old_remote = path.git_origin
 
+      clear_cache
       super(new_user, new_repo)
 
-      @repo_var = nil
-
       new_initial_revision_var = "HOMEBREW_UPDATE_BEFORE#{repo_var}"
       new_current_revision_var = "HOMEBREW_UPDATE_AFTER#{repo_var}"
 

Library/Homebrew/tap.rb

@@ -84,6 +84,7 @@ class Tap
   # clear internal cache
   def clear_cache
     @remote = nil
+    @repo_var = nil
     @formula_dir = nil
     @cask_dir = nil
     @formula_files = nil