Add completions sub-command for generating shell completions (#1386)

Made a quick pr to add a sub-command called completions for generating shell completions, was going pretty crazy that this wasn't a thing :P.

Tried my best to do everything properly, let me know if I need to change something, I tested it and it works perfectly.

Co-authored-by: MrSnowy <snow@mrsnowy.dev>
Reviewed-on: https://git.deuxfleurs.fr/Deuxfleurs/garage/pulls/1386
Reviewed-by: Alex <lx@deuxfleurs.fr>
Co-authored-by: MrSnowy <mrsnowy@noreply.localhost>
Co-committed-by: MrSnowy <mrsnowy@noreply.localhost>

src/garage/cli/local/completions.rs

@@ -0,0 +1,10 @@
+use structopt::{clap::Shell, StructOpt};
+
+use crate::Opt;
+
+pub(crate) fn generate_completions(shell: Shell) {
+	let mut command = Opt::clap();
+
+	let command_name = command.get_name().to_string();
+	command.gen_completions_to(command_name, shell, &mut std::io::stdout());
+}

src/garage/cli/local/mod.rs

@@ -1,3 +1,4 @@
+pub(crate) mod completions;
 pub(crate) mod convert_db;
 pub(crate) mod init;
 pub(crate) mod repair;

src/garage/cli/structs.rs

@@ -1,4 +1,4 @@
-use structopt::StructOpt;
+use structopt::{clap::Shell, StructOpt};
 
 use garage_util::version::garage_version;
 
@@ -77,6 +77,10 @@ pub enum Command {
 		#[structopt(default_value = "null")]
 		payload: String,
 	},
+
+	/// Generate completions for a shell
+	#[structopt(name = "completions", version = garage_version())]
+	Completions { shell: Shell },
 }
 
 // ---------------------------

src/garage/main.rs

@@ -177,6 +177,10 @@ async fn run(opt: Opt) -> Result<(), Error> {
 			);
 			Ok(())
 		}
+		Command::Completions { shell } => {
+			cli::local::completions::generate_completions(shell);
+			Ok(())
+		}
 		_ => cli_command(opt).await,
 	}
 }