[Feature Request] shell completion for bash, zsh, fish

[nurazon59]

Thank you for making this great product!

I would like to request shell completion support for bash, zsh, and fish.

Gitu currently uses clap 4.5.34 with the derive feature, which has built-in support for generating shell completion scripts. However, this functionality is not currently enabled in the codebase.

It would be great if shell completion scripts could be generated and distributed with the project, similar to how other CLI tools handle this.

The implementation would likely involve:

1. Adding a build.rs to generate completion scripts during build
2. Updating documentation to explain how to install completions
3. Or, adding a subcommand to generate completion scripts at runtime (e.g., gitu completion bash)

If accepted, I would like to implement this feature.

[altsem]

I think that sounds good. I'm not sure how the cli will evolve, but if this is an easy addition, then why not!