largefiles: override merge.emptyactions() to include `lfmr`
I found it weird that we were not already doing this. I encountered this while
using `emptyactions()` in mergeresult() class.
Differential Revision: https://phab.mercurial-scm.org/D8835
To run the tests, do:
cd tests/
python run-tests.py
See https://mercurial-scm.org/wiki/WritingTests for
more information on writing tests.