Improve make_playlist.py code quality and fix bugs (#989)

* Improve make_playlist.py code quality and fix bugs

- Fix group name bug: replace underscores with spaces before title()
  so filenames like north_korea.md produce "North Korea" not "North_Korea"
- Fix resource leaks: use context managers for all file handles including
  EPG list and per-country playlist files
- Remove os.chdir() global side effect: use absolute paths derived from
  the script location instead
- Avoid calling to_m3u_line() twice per channel by caching the result
- Fix redundant trailing colon in filename[:-3:] slice
- Consistent use of write() for headers instead of mixing print/write
- Strip blank lines from EPG URL list when reading

* Skip commit and push when playlist has no changes

Prevents the workflow from failing with exit code 1 when the generated
playlist is identical to the previous run and there is nothing to commit.

* Use current branch instead of hardcoded master for push

Replace hardcoded origin/master reference with @{u} (upstream of current
branch) for the diff check, and use HEAD for the push target so the
workflow works correctly on any branch.

---------

Co-authored-by: Kálmán „KAMI” Szalai <kami911gmail.com>

.github/workflows/update_playlist.yml

@@ -17,5 +17,5 @@ jobs:
           git config user.email "playlistbot@users.noreply.github.com" || true
           python3 ./make_playlist.py
           git add .
-          git commit --quiet -m "Update Playlist (GitHub Actions)"
-          git push -f origin master
+          git diff --staged --quiet || git commit --quiet -m "Update Playlist (GitHub Actions)"
+          git diff --quiet HEAD @{u} || git push -f origin HEAD