Fetch required commit before switching

What does this merge request do and why?

This fixes an issue where the current repo does not contain the required tree-sitter commit and git checkout will fail. This issue is blocking the release pipeline.

fatal: reference is not a tree: 16de60e6588ad39afe274b13cd494f97e0f953c7
Traceback (most recent call last):
  File "/builds/gitlab-org/modelops/applied-ml/code-suggestions/ai-assist/scripts/build-tree-sitter-lib.py", line 138, in <module>
Checking out grammars in /builds/gitlab-org/modelops/applied-ml/code-suggestions/ai-assist/scripts/vendor/Linux-5.15.109+-x86_64-with-glibc2.36
Updating tree-sitter-c
Updating tree-sitter-c-sharp
Updating tree-sitter-cpp
Updating tree-sitter-go
Updating tree-sitter-java
Updating tree-sitter-javascript
Updating tree-sitter-kotlin
    sys.exit(main())
  File "/builds/gitlab-org/modelops/applied-ml/code-suggestions/ai-assist/scripts/build-tree-sitter-lib.py", line 108, in main
    subprocess.check_call(["git", "checkout", commit], cwd=name)
  File "/usr/local/lib/python3.9/subprocess.py", line 373, in check_call
    raise CalledProcessError(retcode, cmd)
subprocess.CalledProcessError: Command '['git', 'checkout', '16de60e6588ad39afe274b13cd494f97e0f953c7']' returned non-zero exit status 128.

https://gitlab.com/gitlab-org/modelops/applied-ml/code-suggestions/ai-assist/-/jobs/5513485835

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

Merge request checklist

• Tests added for new functionality. If not, please raise an issue to follow up.
• Documentation added/updated, if needed.

Relates to !456 (merged)