Update scripts/lint-conflicts.sh script

There's two problems with the current script:

1. It's not compatible with the BSD version of grep that ships with OSX:

   gitlab-ee master % ./scripts/lint-conflicts.sh
   grep: invalid option -- z
   usage: grep [-abcDEFGHhIiJLlmnOoqRSsUVvwxZ] [-A num] [-B num] [-C[num]]
           [-e pattern] [-f file] [--binary-files=value] [--color=when]
           [--context[=num]] [--directories=action] [--label] [--line-buffered]
           [--null] [pattern] [file ...]

2. It's missing results sometimes:

   gitlab-ee master % ./scripts/lint-conflicts.sh
   
   gitlab-ee master % echo $?
   0

   vs.

   gitlab-ee master % g grep -E '^<<<<<<< HEAD' -- '*.haml' '*.js' '*.rb'
   app/views/projects/settings/repository/show.html.haml:6:<<<<<<< HEAD

https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/7252 included this change and was apparently missed by the lint, necessitating https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/7253.

I think the git grep command I used above should cover our needs and be platform agnostic. It's also faster and simpler. 🎉