Add --diff option ruff format (#7937)

**Summary** `ruff format --diff` is similar to `ruff format --check`,
but we don't only error with the list of file that would be formatted,
but also show a diff between the unformatted input and the formatted
output.

```console
$ ruff format --diff scratch.py scratch.pyi scratch.ipynb
warning: `ruff format` is not yet stable, and subject to change in future versions.
--- scratch.ipynb
+++ scratch.ipynb
@@ -1,3 +1,4 @@
 import numpy
-maths = (numpy.arange(100)**2).sum()
-stats= numpy.asarray([1,2,3,4]).median()
+
+maths = (numpy.arange(100) ** 2).sum()
+stats = numpy.asarray([1, 2, 3, 4]).median()
--- scratch.py
+++ scratch.py
@@ -1,3 +1,3 @@
 x = 1
-y=2
+y = 2
 z = 3
2 files would be reformatted, 1 file left unchanged
```

With `--diff`, the summary message gets printed to stderr to allow e.g.
`ruff format --diff . > format.patch`.

At the moment, jupyter notebooks are formatted as code diffs, while
everything else is a real diff that could be applied. This means that
the diffs containing jupyter notebooks are not real diffs and can't be
applied. We could change this to json diffs, but they are hard to read.
We could also split the diff option into a human diff option, where we
deviate from the machine readable diff constraints, and a proper machine
readable, appliable diff output that you can pipe into other tools.

To make the tests work, the results (and errors, if any) are sorted
before printing them. Previously, the print order was random, i.e. two
identical runs could have different output.

Open question: Should this go into the markdown docs? Or will this be
subsumed by the integration of the formatter into `ruff check`?

**Test plan** Fixtures for the change and no change cases, including a
jupyter notebook and for file input and stdin.

Fixes #7231

---------

Co-authored-by: Micha Reiser <micha@reiser.io>

crates/ruff_cli/resources/test/fixtures/formatted.py

@@ -0,0 +1 @@
+print("All formatted!")

crates/ruff_cli/resources/test/fixtures/unformatted.ipynb

@@ -0,0 +1,37 @@
+{
+ "cells": [
+  {
+   "cell_type": "code",
+   "execution_count": null,
+   "id": "98e1dd71-14a2-454d-9be0-061dde560b07",
+   "metadata": {},
+   "outputs": [],
+   "source": [
+    "import numpy\n",
+    "maths = (numpy.arange(100)**2).sum()\n",
+    "stats= numpy.asarray([1,2,3,4]).median()"
+   ]
+  }
+ ],
+ "metadata": {
+  "kernelspec": {
+   "display_name": "Python 3 (ipykernel)",
+   "language": "python",
+   "name": "python3"
+  },
+  "language_info": {
+   "codemirror_mode": {
+    "name": "ipython",
+    "version": 3
+   },
+   "file_extension": ".py",
+   "mimetype": "text/x-python",
+   "name": "python",
+   "nbconvert_exporter": "python",
+   "pygments_lexer": "ipython3",
+   "version": "3.10.12"
+  }
+ },
+ "nbformat": 4,
+ "nbformat_minor": 5
+}

crates/ruff_cli/resources/test/fixtures/unformatted.py

@@ -0,0 +1,3 @@
+x = 1
+y=2
+z = 3

crates/ruff_cli/src/args.rs

@@ -353,6 +353,10 @@ pub struct FormatCommand {
     /// files would have been modified, and zero otherwise.
     #[arg(long)]
     pub check: bool,
+    /// Avoid writing any formatted files back; instead, exit with a non-zero status code and the
+    /// difference between the current file and how the formatted file would look like.
+    #[arg(long)]
+    pub diff: bool,
     /// Path to the `pyproject.toml` or `ruff.toml` file to use for configuration.
     #[arg(long, conflicts_with = "isolated")]
     pub config: Option<PathBuf>,
@@ -520,6 +524,7 @@ impl FormatCommand {
         (
             FormatArguments {
                 check: self.check,
+                diff: self.diff,
                 config: self.config,
                 files: self.files,
                 isolated: self.isolated,
@@ -577,6 +582,7 @@ pub struct CheckArguments {
 #[allow(clippy::struct_excessive_bools)]
 pub struct FormatArguments {
     pub check: bool,
+    pub diff: bool,
     pub config: Option<PathBuf>,
     pub files: Vec<PathBuf>,
     pub isolated: bool,