Fixed Repository#exists? to handle errors

Now that Repository#raw_repository no longer sets the autocrlf option it
will also no longer raise any NoRepository errors since it doesn't
access Rugged any more. This also means that Repository#exists? can't
simply return the raw repository as this is no indication of whether or
not the repository actually exists (besides returning a non boolean is
weird in the first place).

To solve this problem Repository#exists? now properly checks if the
repository exists and returns true/false instead of a
Gitlab::Git::Repository or nil object.

app/models/repository.rb

@@ -23,11 +23,7 @@ class Repository
   def raw_repository
     return nil unless path_with_namespace
 
-    @raw_repository ||= begin
-      Gitlab::Git::Repository.new(path_to_repo)
-    rescue Gitlab::Git::Repository::NoRepository
-      nil
-    end
+    @raw_repository ||= Gitlab::Git::Repository.new(path_to_repo)
   end
 
   def update_autocrlf_option
@@ -42,7 +38,10 @@ class Repository
   end
 
   def exists?
-    raw_repository
+    raw_repository.rugged
+    true
+  rescue Gitlab::Git::Repository::NoRepository
+    false
   end
 
   def empty?