Escape the query argument provided to `git grep` by `search_files`

Closes #14963.

app/models/repository.rb

@@ -795,7 +795,7 @@ 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 #{query} #{ref || root_ref})
+    args = %W(#{Gitlab.config.git.bin_path} grep -i -I -n --before-context #{offset} --after-context #{offset} -e #{Regexp.escape(query)} #{ref || root_ref})
     Gitlab::Popen.popen(args, path_to_repo).first.scrub.split(/^--$/)
   end
 

spec/models/repository_spec.rb

@@ -94,6 +94,12 @@ describe Repository, models: true do
 
     it { is_expected.to be_an Array }
 
+    it 'regex-escapes the query string' do
+      results = repository.search_files("test\\", 'master')
+
+      expect(results.first).not_to start_with('fatal:')
+    end
+
     describe 'result' do
       subject { results.first }