ptgrep / ptig - -l, --files-with-matches - only searches for filenames

[clach04]

Search only file name support is great... but it's enabled with the regular grep flags:

-l, --files-with-matches  print only names of FILEs containing matches

TODO

1. ✅ new flag for search only filenames, specific to ptgrep / ptig - based on https://github.com/clach04/puren_tonbo/wiki/grep%E2%80%90like-tools
   • short -y would not collide
   • long --find_only_filename would match internal search() API, --search-filenames-only
2. ✅ implement real support