cli/diff: Deprecation Warning

This command is just a small wrapper around `git diff` not worth
maintaining.

CHANGELOG.md

@@ -12,6 +12,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ## Deprecations
 
 - The option `rad self --nid` was deprecated in favor of `rad status --only nid`
+- `rad diff` was deprecated in favor of using `git diff`
+
 ## New Features
 
 - `rad clone` now supports the flag `--bare` which works analoguously to

crates/radicle-cli/examples/rad-diff.md

@@ -1,3 +1,8 @@
+``` (stderr)
+$ rad diff
+! Deprecated: The command/option `rad diff` is deprecated and will be removed. Please use `git diff` instead.
+```
+
 Exploring `rad diff`.
 
 ``` ./main.c

crates/radicle-cli/src/commands/diff.rs

@@ -82,6 +82,8 @@ impl Args for Options {
 }
 
 pub fn run(options: Options, _ctx: impl term::Context) -> anyhow::Result<()> {
+    crate::warning::deprecated("rad diff", "git diff");
+
     let repo = rad::repo()?;
     let oids = options
         .commits