commit: force create a new filenode if it was set in mergestate by merge
For reasons mentioned in previous commits, we will like to forcefully create a
new filenode sometimes.
Combination of this patch and previous one, we fixed a case in
`test-merge-combination.t`.
This does not yet results in conflict where it should, I need to investigate
more about what's happening as it should be a change-delete conflict now.
Differential Revision: https://phab.mercurial-scm.org/D8989