Allow for overriding only specific Mustache templates/partials. (#1719)

* Allow for overriding only specific templates/partials. Resolves  #1450.

test/examples/template_cascade/navbar.mustache

@@ -0,0 +1,8 @@
+<nav>
+  <div style="background-color: red;">NAVBAR_OVERRIDE</div>
+
+  <p>Still include an original partial to ensure the fallback works even when nested from an overriden partial:</p>
+  <div>
+    {{>mobilenav}}
+  </div>
+</nav>

test/examples/template_cascade/page.mustache

@@ -0,0 +1,12 @@
+<div id="wiki-wrapper" class="page">
+
+<p>Include an overridden partial:</p>
+<div id="head">
+	{{#navbar?}}{{>navbar}}{{/navbar?}}
+</div>
+
+<div style="background-color: red;">PAGE_OVERRIDE</div>
+
+{{>wiki_content}}
+
+</div>

test/test_template_cascade.rb

@@ -0,0 +1,43 @@
+# ~*~ encoding: utf-8 ~*~
+require File.expand_path(File.join(File.dirname(__FILE__), 'helper'))
+
+class TestTemplateCascade < Minitest::Unit::TestCase
+  include Rack::Test::Methods
+
+  def setup
+    @path = cloned_testpath('examples/lotr.git')
+    Precious::App.set(:gollum_path, @path)
+    Precious::App.set(:wiki_options, {template_dir: testpath('examples/template_cascade')})
+    @wiki = Gollum::Wiki.new(@path)
+  end
+
+  def teardown
+    FileUtils.rm_rf(@path)
+  end
+
+  def app
+    Precious::App
+  end
+
+  def test_overridden_page_template_is_used
+    get '/Home'
+
+    assert last_response.body.include?('PAGE_OVERRIDE')
+  end
+
+  def test_overridden_navbar_partial_is_used
+    get '/Home'
+
+    assert last_response.body.include?('NAVBAR_OVERRIDE')
+  end
+
+  def test_overridden_templates_are_ignore_without_template_dir_set
+    Precious::App.set(:wiki_options, {template_dir: nil})
+
+    get '/Home'
+    assert_equal '/Home', last_request.fullpath
+    assert last_response.ok?
+    assert_no_match /PAGE_OVERRIDE/, last_response.body
+    assert_no_match /NAVBAR_OVERRIDE/, last_response.body
+  end
+end