adding --branches-of-interest flag to diff_root_files.py

This adds a flag to the diff_root_files.py to compare only specific branches to speed up the comparison process.

In a test comparing a file with itself, there's a huge time difference between comparing all branches and comparing only a single branch: comparing all branches takes 450s for 25 events and 900s for 50 evts, comparing a single branch only takes 33s for 25evts and 39s for 50evts.

I'd like to point out that you can only filter BRANCHES as they are retrieved here:

branches = [b.GetName() for b in tree.GetListOfBranches()]

The script reports if any branches are matching with the regex provided to the flag.