Add Project.reference_for_path

It mirrors the functionality of the method with
the same name in CocoaPod's Pod::Project class
(which after this patch will override it). The
differences are 1) CocoaPod's implementation is
constant time but this is linear wrt the number
of objects, 2) CocoaPod's implementation does not
work when you load a previously existing file.

lib/xcodeproj/project.rb

@@ -469,6 +469,24 @@ def files
       objects.select { |obj| obj.class == PBXFileReference }
     end
 
+    # Returns the file reference for the given absolute path.
+    #
+    # @param  [#to_s] absolute_path
+    #         The absolute path of the file whose reference is needed.
+    #
+    # @return [PBXFileReference] The file reference.
+    # @return [Nil] If no file reference could be found.
+    #
+    def reference_for_path(absolute_path)
+      unless Pathname.new(absolute_path).absolute?
+        raise ArgumentError, "Paths must be absolute #{absolute_path}"
+      end
+
+      objects.find do |child|
+        child.isa == 'PBXFileReference' && child.real_path == absolute_path
+      end
+    end
+
     # @return [ObjectList<PBXNativeTarget>] A list of all the targets in the
     #         project.
     #

spec/project_spec.rb

@@ -291,6 +291,19 @@ module ProjectSpecs
         @project.files.should.include?(f)
       end
 
+      it "finds file references by absolute path" do
+        file_path = 'Classes/Test.h'
+        @project.reference_for_path(@project.path.dirname + file_path).should.be.nil
+        file = @project.new_file(file_path)
+        @project.reference_for_path(@project.path.dirname + file_path).should == file
+      end
+
+      it "does not find references by relative path" do
+        should.raise ArgumentError do
+          @project.reference_for_path(@project.path.basename)
+        end.message.should.match /must be absolute/
+      end
+
       it "returns the targets" do
         target = @project.new_target(:static_library, 'Pods', :ios).product_reference
         @project.products.should.include?(target)