Adds ability to link to all files

Fixes request made in #4624 and bug found in #5182

lib/jekyll/tags/link.rb

@@ -16,8 +16,8 @@ module Jekyll
       def render(context)
         site = context.registers[:site]
 
-        site.docs_to_write.each do |document|
+        site.each_site_file do |item|
-          return document.url if document.relative_path == @relative_path
+          return item.url if item.relative_path == @relative_path
         end
 
         raise ArgumentError, <<eos

test/source/info.md

@@ -0,0 +1,7 @@
+---
+title: Information
+---
+
+# Information
+
+Very important information is here

test/test_tags.rb

@@ -12,6 +12,7 @@ class TestTags < JekyllUnitTest
 
     site.posts.docs.concat(PostReader.new(site).read_posts("")) if override["read_posts"]
     CollectionReader.new(site).read if override["read_collections"]
+    site.read if override["read_all"]
 
     info = { :filters => [Jekyll::Filters], :registers => { :site => site } }
     @converter = site.converters.find { |c| c.class == converter_class }
@@ -627,6 +628,36 @@ CONTENT
     end
   end
 
+  context "simple page with linking to a page" do
+    setup do
+      content = <<CONTENT
+---
+title: linking
+---
+
+{% link contacts.html %}
+{% link info.md %}
+CONTENT
+      create_post(content, {
+        "source"           => source_dir,
+        "destination"      => dest_dir,
+        "read_all"         => true
+      })
+    end
+
+    should "not cause an error" do
+      refute_match(%r!markdown\-html\-error!, @result)
+    end
+
+    should "have the URL to the \"contacts\" item" do
+      assert_match(%r!/contacts\.html!, @result)
+    end
+
+    should "have the URL to the \"info\" item" do
+      assert_match(%r!/info\.html!, @result)
+    end
+  end
+
   context "simple page with linking" do
     setup do
       content = <<CONTENT