largefiles: introduce a constant for 'lfmr' action It's better to use a dedicated constant instead of a string which makes pretty less sense. Differential Revision: https://phab.mercurial-scm.org/D8836

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

merge: unify logic of couple of if-else's in manifestmerge() Right now manifestmerge() contains very nested if-else conditions and it's not easy to understand what is happening. I was looking for easy unifications and found these two. Differential Revision: https://phab.mercurial-scm.org/D8834