Don't check for commit existence in Github::Representation::Branch#exists?

We don't need to check if the existing branch contains a specific SHA
since we only use the branch's name, the SHA is taken from the pull
request raw `sha` data.
Signed-off-by: Rémy Coutable <remy@rymai.me>

lib/github/representation/branch.rb

@@ -26,7 +26,9 @@ module Github
       end
 
       def exists?
-        @exists ||= branch_exists? && commit_exists?
+        return @exists if defined?(@exists)
+
+        @exists = repository.branch_exists?(ref)
       end
 
       def valid?
@@ -47,14 +49,6 @@ module Github
 
       private
 
-      def branch_exists?
-        repository.branch_exists?(ref)
-      end
-
-      def commit_exists?
-        repository.branch_names_contains(sha).include?(ref)
-      end
-
       def repository
         @repository ||= options.fetch(:repository)
       end

lib/github/representation/pull_request.rb

@@ -27,6 +27,8 @@ module Github
       end
 
       def target_branch_exists?
+        return @target_branch_exists if defined?(@target_branch_exists)
+
         @target_branch_exists ||= target_branch.exists?
       end