Update diff commands in UMDP3 fixer

@Pierre-siddall y travaille déjà.

Depuis le 19/3/2026.

UMDP3 fixer currently uses fcm diff and bdiff commands to take the difference between the equivalent of the main branch and the developers branch for example in this statement:

```Python
def get_branch_diff():
"""If in a local working copy of an FCM branch, return a
list of files returned by running a branch-diff command"""

# Use the bdiff command to extract a list of files that have changed
# on the user's branch
bdiff = subprocess.Popen(
"fcm bdiff --summarize", stdout=subprocess.PIPE, shell=True
)

bdiff_stdout, _ = bdiff.communicate()
bdiff_stdout = bdiff_stdout.decode(sys.stdout.encoding)
```
Therefore UMDP3 fixer needs to be refactored to reflect the move to GitHub.