`Programming exercises`: Generate the git-diff report if it does not exist already

[Hialus]

Checklist

General

• [x] I tested all changes and their related features with all corresponding user types on a test server.
• [x] This is a small issue that I tested locally and was confirmed by another developer on a test server.
• [x] Language: I followed the guidelines for inclusive, diversity-sensitive, and appreciative language.
• [x] I chose a title conforming to the naming conventions for pull requests.

Server

• [x] I followed the coding and design guidelines.
• [x] I added multiple integration tests (Spring) related to the features (with a high test coverage).
• [x] I added @PreAuthorize and checked the course groups for all new REST Calls (security).
• [x] I implemented the changes with a good performance and prevented too many database calls.
• [x] I documented the Java code using JavaDoc style.

Changes affecting Programming Exercises

• [x] I tested all changes and their related features with all corresponding user types on Test Server 1 (Atlassian Suite).
• [x] I tested all changes and their related features with all corresponding user types on Test Server 2 (Jenkins and Gitlab).

Motivation and Context

Description

Steps for Testing

Prerequisites:

• 1 Instructor

1. Log in to Artemis
2. Do one of these:
   • Create a programming exercise (and maybe wait for the build to finish)
   • Import a programming exercise (and maybe wait for the build to finish)
   • Locally: Delete the git-diff report of a programming exercise in the database
3. Go to the exercise detail page
4. The git-diff report should be (re)generated and shown at the bottom

Review Progress

Code Review

• [x] Review 1
• [ ] Review 2

Manual Tests

• [ ] Test 1
• [ ] Test 2

Test Coverage

Unchanged

Screenshots

The git-diff stats that should be visible: