Fix #53298 JupyterHub restarts should work without errors

What does this MR do?

Before this change

When user starts Jupyter after a period of inactivity, it fails with 500. This happens because the PostStart hook (git clone statement) fails since the repo directory exists.

After this change

When user starts Jupyter after a period of inactivity, it starts normally. We changed the git clone statements in a way that it returns exit code 0 even in the problematic case.

What are the relevant issue numbers?

Fix #53298 (closed)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.