Memoize all of the significant calls in Git::Status

When the status has many entries, there were substantial inefficiencies
in this class - calling predicates like `changed?(filename)` would
iterate the status, constructing a transient `changed` subhash, then
test that subhash to see if the file in question was in it (for
example).

After this, it will _keep_ those sub-hashes for reuse on the Status
instance, as well as downcased versions if they happen to get requested
(by case-insensitive calls).

lib/git/status.rb

@@ -22,7 +22,7 @@ def initialize(base)
     #
     # @return [Enumerable]
     def changed
-      @files.select { |_k, f| f.type == 'M' }
+      @_changed ||= @files.select { |_k, f| f.type == 'M' }
     end
 
     #
@@ -34,19 +34,15 @@ def changed
     #     changed?('lib/git.rb')
     # @return [Boolean]
     def changed?(file)
-      if ignore_case?
-        changed.keys.map(&:downcase).include?(file.downcase)
-      else
-        changed.member?(file)
-      end
+      case_aware_include?(:changed, :lc_changed, file)
     end
 
     # Returns an Enumerable containing files that have been added.
     # File path starts at git base directory
     #
     # @return [Enumerable]
     def added
-      @files.select { |_k, f| f.type == 'A' }
+      @_added ||= @files.select { |_k, f| f.type == 'A' }
     end
 
     # Determines whether the given file has been added to the repository
@@ -58,11 +54,7 @@ def added
     #     added?('lib/git.rb')
     # @return [Boolean]
     def added?(file)
-      if ignore_case?
-        added.keys.map(&:downcase).include?(file.downcase)
-      else
-        added.member?(file)
-      end
+      case_aware_include?(:added, :lc_added, file)
     end
 
     #
@@ -71,7 +63,7 @@ def added?(file)
     #
     # @return [Enumerable]
     def deleted
-      @files.select { |_k, f| f.type == 'D' }
+      @_deleted ||= @files.select { |_k, f| f.type == 'D' }
     end
 
     #
@@ -83,11 +75,7 @@ def deleted
     #     deleted?('lib/git.rb')
     # @return [Boolean]
     def deleted?(file)
-      if ignore_case?
-        deleted.keys.map(&:downcase).include?(file.downcase)
-      else
-        deleted.member?(file)
-      end
+      case_aware_include?(:deleted, :lc_deleted, file)
     end
 
     #
@@ -96,7 +84,7 @@ def deleted?(file)
     #
     # @return [Enumerable]
     def untracked
-      @files.select { |_k, f| f.untracked }
+      @_untracked ||= @files.select { |_k, f| f.untracked }
     end
 
     #
@@ -108,11 +96,7 @@ def untracked
     #     untracked?('lib/git.rb')
     # @return [Boolean]
     def untracked?(file)
-      if ignore_case?
-        untracked.keys.map(&:downcase).include?(file.downcase)
-      else
-        untracked.member?(file)
-      end
+      case_aware_include?(:untracked, :lc_untracked, file)
     end
 
     def pretty
@@ -290,5 +274,33 @@ def ignore_case?
     rescue Git::FailedError
       @_ignore_case = false
     end
+
+    def downcase_keys(hash)
+      hash.map { |k, v| [k.downcase, v] }.to_h
+    end
+
+    def lc_changed
+      @_lc_changed ||= changed.transform_keys(&:downcase)
+    end
+
+    def lc_added
+      @_lc_added ||= added.transform_keys(&:downcase)
+    end
+
+    def lc_deleted
+      @_lc_deleted ||= deleted.transform_keys(&:downcase)
+    end
+
+    def lc_untracked
+      @_lc_untracked ||= untracked.transform_keys(&:downcase)
+    end
+
+    def case_aware_include?(cased_hash, downcased_hash, file)
+      if ignore_case?
+        send(downcased_hash).include?(file.downcase)
+      else
+        send(cased_hash).include?(file)
+      end
+    end
   end
 end