Use system instead of backticks for git command

Additionally we bypass the shell by splitting the args into tokens.
This has the effect of quoting 'locale/*/gitlab.po' which means that git
will only match against the working tree. So as we no longer attempt to
checkout a file that is unknown to git, the git error seen in
https://gitlab.com/gitlab-org/gitlab/-/issues/238208#note_407172042
is resolved.

lib/tasks/gettext.rake

@@ -38,7 +38,7 @@ namespace :gettext do
     Rake::Task['gettext:find'].invoke
 
     # leave only the required changes.
-    unless system('git checkout -- locale/*/gitlab.po')
+    unless system(*%w(git checkout -- locale/*/gitlab.po))
       raise 'failed to cleanup generated locale/*/gitlab.po files'
     end