Fix broken external links in help/index.html

An external link was recently added but was broken because
'https://gitlab.com/help/' was prepended to every link in the page.

Since no link in the main help readme begins with "help" and since doing
so wouldn't make sense, the substitution conditionaly prepending "help"
can be simplified and reused.
Signed-off-by: David Wagner <david@marvid.fr>

app/controllers/help_controller.rb

@@ -6,9 +6,9 @@ class HelpController < ApplicationController
   def index
     @help_index = File.read(Rails.root.join('doc', 'README.md'))
 
-    # Prefix Markdown links with `help/` unless they already have been
-    # See http://rubular.com/r/ie2MlpdUMq
-    @help_index.gsub!(/(\]\()(\/?help\/)?([^\)\(]+\))/, '\1/help/\3')
+    # Prefix Markdown links with `help/` unless they are external links
+    # See http://rubular.com/r/MioSrVLK3S
+    @help_index.gsub!(%r{(\]\()(?!.+://)([^\)\(]+\))}, '\1/help/\2')
   end
 
   def show

spec/controllers/help_controller_spec.rb

@@ -16,14 +16,6 @@ describe HelpController do
       end
     end
 
-    context 'when url prefixed with help/' do
-      it 'will be an absolute path' do
-        stub_readme("[API](help/api/README.md)")
-        get :index
-        expect(assigns[:help_index]).to eq '[API](/help/api/README.md)'
-      end
-    end
-
     context 'when url prefixed with help' do
       it 'will be an absolute path' do
         stub_readme("[API](helpful_hints/README.md)")
@@ -32,11 +24,11 @@ describe HelpController do
       end
     end
 
-    context 'when url prefixed with /help/' do
+    context 'when url is an external link' do
       it 'will not be changed' do
-        stub_readme("[API](/help/api/README.md)")
+        stub_readme("[external](https://some.external.link)")
         get :index
-        expect(assigns[:help_index]).to eq '[API](/help/api/README.md)'
+        expect(assigns[:help_index]).to eq '[external](https://some.external.link)'
       end
     end
   end