Fix reset TP

Il faut fixer cette erreur lors du reset d'un TP par rapport à git.

Internal Server Error: /practical-work/16/reset_groups/

GitCommandError at /practical-work/16/reset_groups/
Cmd('git') failed due to: exit code(1)
  cmdline: git checkout master
  stderr: 'error: Your local changes to the following files would be overwritten by checkout:
        xxx/progress.json
        xxx/src/file_de_priorite.py
        zzz/progress.json
        zzz/src/file_de_priorite.py
Please commit your changes or stash them before you switch branches.
Aborting'