Use extended regexp flag for git grep-ing files

Closes #3055

app/models/repository.rb

@@ -854,7 +854,7 @@ class Repository
 
   def search_files(query, ref)
     offset = 2
-    args = %W(#{Gitlab.config.git.bin_path} grep -i -I -n --before-context #{offset} --after-context #{offset} -e #{Regexp.escape(query)} #{ref || root_ref})
+    args = %W(#{Gitlab.config.git.bin_path} grep -i -I -n --before-context #{offset} --after-context #{offset} -E -e #{Regexp.escape(query)} #{ref || root_ref})
     Gitlab::Popen.popen(args, path_to_repo).first.scrub.split(/^--$/)
   end
 

spec/models/repository_spec.rb

@@ -100,6 +100,12 @@ describe Repository, models: true do
       expect(results.first).not_to start_with('fatal:')
     end
 
+    it 'properly handles an unmatched parenthesis' do
+      results = repository.search_files("test(", 'master')
+
+      expect(results.first).not_to start_with('fatal:')
+    end
+
     describe 'result' do
       subject { results.first }