Merge branch '24008-use-git-grep-for-lint-conflicts' into 'master'

Use git grep to search for conflict headers

Closes #24008

See merge request gitlab-org/gitlab!20937

scripts/lint-conflicts.sh

 #!/bin/sh
-output=`git ls-files -z | grep -zvE '\.(rb|js|haml)$' | xargs -0n1 grep -HEn '^<<<<<<< '`
+output=`git grep -En '^<<<<<<< ' -- . ':(exclude)spec/lib/gitlab/conflict/file_spec.rb' ':(exclude)spec/lib/gitlab/git/conflict/parser_spec.rb'`
 echo $output
 test -z "$output"