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
ERROR: FIXME: This is a known failure of filelog.size for data starting with \1\n
OK.