feat(cli): add completion subcommand for shell completion scripts

[ChrisJr404]

Closes #544.

Adds a completion subcommand that prints a shell completion script to stdout, generated with clap_complete:

gitu completion bash > ~/.local/share/bash-completion/completions/gitu

Supported shells are bash, elvish, fish, powershell and zsh (whatever clap_complete::Shell offers). Since generating a script doesn't need a terminal or a git repository, it's handled early in main, right alongside --version, and returns before any of that setup runs.

Changes

• src/cli.rs: new Commands::Completion { shell } variant and a small completions(shell, out) helper that writes the script to any Write (keeps the generation testable and lets main pass stdout).
• src/main.rs: handle the subcommand before terminal/repo setup.
• src/app.rs: the completion arm is unreachable! here since it never reaches app construction.
• Docs: a "Shell completions" section in docs/installing.md with per-shell install paths, plus a one-line pointer from the README.

Tests

Added src/tests/completion.rs covering: the subcommand parses, an unknown shell is rejected, and every supported shell produces a non-empty script that references the binary name.

test tests::completion::completion_rejects_unknown_shell ... ok
test tests::completion::completion_subcommand_parses ... ok
test tests::completion::generates_non_empty_script_for_every_shell ... ok

cargo fmt --check and cargo clippy -- -Dwarnings are clean. (The pre-existing fetch/pull/push snapshot tests fail in my sandbox on a clean master too — they're unrelated to this change.)

Happy to adjust the subcommand name or wire the scripts into the release artifacts instead if you'd prefer that approach.