Currently, gitui prioritizes file extensions for syntax detection. When a file lacks an extension (e.g. Makefile), or the extension isn't tied to a specific format (e.g. .lock), it disables syntax highlighting.

This PR changes the syntax detecton method to the entire filename and the first line of the file using find_syntax_for_file().

I followed the checklist:

• I added unittests
• I ran make check without errors
• I tested the overall application
• I added an appropriate item to the changelog

Screenshots of highlighting Cargo.lock and Makefile from this repository: