Add an option to use `unidiff` format for diff API responses

What does this MR do and why?

Contriubutes to #23284 (closed)

Problem

diff field doesn't return diff headers compatible with Unified format.

Solution

Use Unified format for diffs when unidiff option is provided. It should keep the backward compatibility with existing API.

Unified format: https://www.gnu.org/software/diffutils/manual/html_node/Detailed-Unified.html

The current version of diff output is missing two headers:

--- a/test.yml
+++ b/test.yml

Screenshots or screen recordings

http://127.0.0.1:3000/api/v4/projects/1/repository/commits/HEAD/diff

http://127.0.0.1:3000/api/v4/projects/1/repository/commits/HEAD/diff?unidiff=true

How to set up and validate locally

1. Request diff for a commit in current diff format: http://127.0.0.1:3000/api/v4/projects/2/repository/commits/HEAD/diff
2. Request diff for a commit in unidiff format: http://127.0.0.1:3000/api/v4/projects/2/repository/commits/HEAD/diff?unidiff=true

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

doc/api/commits.md

@@ -452,7 +452,7 @@ Parameters:
 | --------- | ---- | -------- | ----------- |
 | `id`      | integer/string | yes | The ID or [URL-encoded path of the project](rest/index.md#namespaced-path-encoding) owned by the authenticated user
 | `sha` | string | yes | The commit hash or name of a repository branch or tag |
-| `unidiff` | boolean | no | The diff will be in unidiff format |
+| `unidiff` | boolean | no | The diff will be in Unified diff format |
 
 ```shell
 curl --header "PRIVATE-TOKEN: <your_access_token>" "https://gitlab.example.com/api/v4/projects/5/repository/commits/master/diff"