Rename classes DiffOp_* to DiffOp*

Part of program to remove underscores from class names. Checked core and
600+ extensions for occurrences. All uses are in core in core are updated
in this patch. No uses in extensions.

Change-Id: I86b8c6f8702e661554c7b794df09892db94a84d1
Merge "Rename classes DiffOp_* to DiffOp*"