feat(config): add show_root_path option for panel headers

[jesses-code-adventures]

Adds show_root_path as a boolean option in the file_panel configuration, allowing a user to hide the launch directory at the top of the file panel by passing false.

[dlyongemallo]

Thanks for the PR!

The files doc/diffview.txt and doc/diffview_defaults.txt should be updated, per the contributing guidelines.

Could make the title a bit more specific: feat(config): ....

Is it intentional to gate this only to the file panel? The file history panel also shows the root path, and it makes sense for a top-level option (i.e., conf.show_root_path) to apply to both. Alternatively, it could remain under file_panel with the equivalent option under file_history_panel, but I don't think there's a real use case for hiding it in one panel but not the other.

[jesses-code-adventures]

My pleasure @dlyongemallo ! Thanks for taking over maintenance of this project, really appreciate your work. I've applied the changes you requested, let me know if there's anything else.

[dlyongemallo]

@jesses-code-adventures I've merged it. Thanks again for the contribution!