Fix markdown renderer

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

app/helpers/gitlab_markdown_helper.rb

@@ -166,14 +166,14 @@ module GitlabMarkdownHelper
 
   def file_exists?(path)
     return false if path.nil? || path.empty?
-    return @repository.blob_at(current_ref, path).present? || @repository.tree(:head, path).entries.any?
+    return @repository.blob_at(current_sha, path).present? || @repository.tree(current_sha, path).entries.any?
   end
 
   # Check if the path is pointing to a directory(tree) or a file(blob)
   # eg. doc/api is directory and doc/README.md is file
   def local_path(path)
-    return "tree" if @repository.tree(:head, path).entries.any?
-    return "raw" if @repository.blob_at(current_ref, path).image?
+    return "tree" if @repository.tree(current_sha, path).entries.any?
+    return "raw" if @repository.blob_at(current_sha, path).image?
     return "blob"
   end
 
@@ -181,6 +181,14 @@ module GitlabMarkdownHelper
     @commit.nil? ? "master" : @commit.id
   end
 
+  def current_sha
+    if @commit
+      @commit.id
+    else
+      @repository.head_commit.sha
+    end
+  end
+
   # We will assume that if no ref exists we can point to master
   def correct_ref(ref)
     ref ? ref : "master"