Use File.exist? to check if a repository exists

Initializing Rugged objects is way too expensive just to check if a
repository exists. Even though we cache this data once in a while we
have to refresh this. On GitLab.com we have seen Repository#exists?
taking up to _1 minute_ to complete in the absolute worst case, though
usually it sits around a second or so.

Using File.exist? to instead check if $GIT_DIR/refs exists is a much
faster way of checking if a repository was initialized properly.

app/models/repository.rb

@@ -41,11 +41,7 @@ class Repository
     return @exists unless @exists.nil?
 
     @exists = cache.fetch(:exists?) do
-      begin
-        raw_repository && raw_repository.rugged ? true : false
-      rescue Gitlab::Git::Repository::NoRepository
-        false
-      end
+      refs_directory_exists?
     end
   end
 
@@ -1148,6 +1144,12 @@ class Repository
 
   private
 
+  def refs_directory_exists?
+    return false unless path_with_namespace
+
+    File.exist?(File.join(path_to_repo, 'refs'))
+  end
+
   def cache
     @cache ||= RepositoryCache.new(path_with_namespace, @project.id)
   end

spec/models/repository_spec.rb

@@ -811,8 +811,7 @@ describe Repository, models: true do
     end
 
     it 'returns false when a repository does not exist' do
-      expect(repository.raw_repository).to receive(:rugged).
-        and_raise(Gitlab::Git::Repository::NoRepository)
+      allow(repository).to receive(:refs_directory_exists?).and_return(false)
 
       expect(repository.exists?).to eq(false)
     end