Update diff commands in UMDP3 fixer

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.